# The Use of Flocculants and Coagulants to Aid the Settlement of Suspended Sediment in Earthworks Runoff : Trials, Methodology and Design [draft]

June 2004 Technical Publication 227

Auckland Regional Council Technical Publication No. 227, June 2004 ISSN 1175-205X ISBN 1-877353-31-0 Printed on recycled paper2509784 Rev A ■ L1:90873 

#  report 

# The Use of Flocculants and Coagulants to Aid the Settlement of Suspended Sediment in Earthworks Runoff – Trials, Methodology and Design

Auckland Regional CouncilThe Use of Flocculants and Coagulants to Aid The Settlement of Suspended Sediment in Earthworks Runoff – Trials, Methodology and Design

2509784 Beca Carter Hollings & Ferner Ltd Page i L1:90873-flocculation report - Becas Rev A 

# Table of Contents

LIST OF FIGURES..............................................................................................................(i v) LIST OF TABLES ...............................................................................................................(i v) AKNOWLEDGEME NTS ......................................................................................................(v) ABST RACT....................................................................................................................... ..(vi)

1 INTRODUCTION ........................................................................................................... 1
1.1 ALPURT Trials...........................................................................................1 1.2 Greenhithe Trials .......................................................................................2



### 2 CHEMICAL FLOCCULANTS AND COAGUL ANTS ..................................................... 3
2.1 Introduction................................................................................................3 2.2 Coagulation ................................................................................................3 2.3 Flocculation ................................................................................................4 2.4 Polyelectrolyte (Polymer or Polyacrylamide) Flocculants...................4 2.5 Aluminium Coagulants ............................................................................6



### 3 DESIGN OF A LIQUID CHE MICAL DOSING SYSTEM................................................ 8
3.1 Introduction................................................................................................8 3.2 Design of Rainfall Driv en Dosing System..............................................8 3.3 Set Up and Servicing of the Rainfall Driven Dosing System.............14



### 4 THE ALPURT TRIALS ................................................................................................ 15
4.1 Introduction..............................................................................................15 4.2 Polyaluminium Chloride (PAC) Trial ..................................................16



### 5 PRELIMINARY FLOC BLOCK TR IALS...................................................................... 21
5.1 ALPURT Floc Block Trial .......................................................................21 5.2 Greenhithe Floc Block Trial....................................................................23 5.3 Discussion.................................................................................................30 5.4 Characteristics of Floc Block in Stormwater Treatment Conditions 30 5.5 Potential Performance.............................................................................31 5.6 Optimal Flow for Floc Block Treatment...............................................31



### 6 SITUATIONS WHERE CHEMICAL TREATMENT MAY NEED TO BE CONSIDERED33

7 SEDIMENT RETENTION POND DESIGN WITH ALUMINUM COAGULANT TREATMENT

............................................................................................ Error! Bookmark not defined.The Use of Flocculants and Coagulants to Aid The Settlement of Suspended Sediment in Earthworks Runoff – Trials, Methodology and Design

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### 8 SUMMARY .................................................................................................................. 35
8.1 Advantages Of Aluminium Coagulant Treatment.............................35 8.2 Advantages of the Use of Floc Blocks...................................................35The Use of Flocculants and Coagulants to Aid The Settlement of Suspended Sediment in Earthworks Runoff – Trials, Methodology and Design

2509784 Beca Carter Hollings & Ferner Ltd Page iii L1:90873-flocculation report - Becas Rev A 

# List of Figures
Figure 1 Chemical Settling of Colloidal Matter Figure 2 Colloidal Linking Mechanism Figure 3 Rainfall Activated Flocculation System Housing Detail - Side View Figure 4 Rainfall Activate Flocculation Housing Figure 5 Rainfall Activate Flocculation System Catchment Tray – Plan View Figure 6 Rainfall Catchment Tray Figure 7 Rainfall Activated Flocculation Dosing Detail Figure 8 Floc Blocks and Flume Detail Figure 9 Floc Blocks within Channel between Forebay and Pond Figure 10 Pond Inlet Channel full of Sediment



# List of Tables
Table 1 Rainfall Catchment Tray Area Re quired for Different PAC Dose Rates
Table 2 Results of Alum Addition to Limestone Soil Runoff to Evaluate Effects off Alum Overdose Table 3 pH Data for Alum an d PAC Treated Runoff Samples Table 4 Suspended Solids Removal from PAC Treated Stormwater Table 5 Inflows and Discharges of PAC Treated Ponds Table 6 PAC Dosed Sediment Retention Ponds Monitoring Data, 21/10/99
Table 7 PAC Dosed Sediment Re tention Ponds Discharge Data Table 8 Floc Block Trial Discharge Data, 20/01/00 Table 9 Floc Block Trial Discharge Data, 27/01/00 Table 10 Floc Block Trial Discharge Data, 24/02/00 Table 11 Floc Block Trial Discharge Data, 25/04/00 Table 12 Floc Block Trial Discharge Data, 12/05/00 Table 13 Floc Block Trial Discharge Data, 30/05/00 Table 14 Floc Block Trial Discharge Data, 29/06/00The Use of Flocculants and Coagulants to Aid The Settlement of Suspended Sediment in Earthworks Runoff – Trials, Methodology and Design

2509784 Beca Carter Hollings & Ferner Ltd Page iv L1:90873-flocculation report - Becas Rev A Table 15 Advantages of PAC Treatment of Earthworks Runoff for Standard Catchments on an Earthworks Season Basis



# Acknowledgements
Auckland Regional Council wish to acknowledge:



#  Transit New Zealand for largely funding the research, and allowing the use of their construction site;


#  Michael Larcombe for the completeing the field work, drafting the technical publication, and for the use of the photos;


#  Universal Homes Limited for the use of their construction site in Greenhithe; and


#  Ross Winter (Beca Carter Hollings and Ferner Ltd) for the use of figures 3, 5 and 7. Beca Carter Hollings and Ferner also edited and completed the technical publication.The Use of Flocculants and Coagulants to Aid Suspended Sediment Settlement in Earthworks Runoff - Trials, Methodology and Design

2509784 Beca Carter Hollings & Ferner Ltd Page 1 L1:90873-flocculation report - Becas Rev A 

# 1 INTRODUCTION
Land contouring for urbanisation and infrastruc ture development has long been features of the Auckland Region. The Auckland region is also characterised by steep terrain, erosive soils, and high intensity rainfall, which ensures that erosion, and the subsequent sedimentation of freshwater and marine ecosyste ms is an unwanted but inextricable part of the development cycle. Historically, erosion and sediment control measures have been used to minimise the effects of development. However, the escalating scale of development has created a growing awareness of the cumulative sediment relate d ecosystem impacts of many land-disturbing activities occurring within developing areas. Consequently, investigations have been undertaken to improve the existing control me asures and construction methodologies, and develop new control measures. This report presents the results of trials unde rtaken to determine the usefulness of chemical flocculants and coagulants in improving the settlement of suspended sediment contained in sediment laden runoff from earthworks sites, and provides the design of a system for chemical treatment of this runoff. Flocculant s and coagulants act by destabilising particles that would otherwise not settle, or settle very slowly due to their size and electrical charge which cause them to repel each other. Trials were undertaken utilising liquid and solid forms of flocculant chemicals. Initially these trials occurred during the development of the motorway north of Auckland from Albany to Puhoi (ALPURT) and on a small residential development where earthworks were being undertaken in Greenhithe, on Auckland’s north shore.



### 1 . 1 A L P U R T T r i a l s The ALPURT motorway extension development included significant earthworks in the catchments of sensitive stream systems. Du e to the scale of the works and the potential effects of a number of sediment laden discha rges entering these stream systems, it was determined via the resource consent process, that flocculation systems should be investigated for the purpose of increasing the effectiveness of sediment retention measures. On this basis, a system utilising solid floccu lation chemical as well as a rain activated system utilising fluid chemical were developed. Allied Colloids (Australia Pty Ltd.), throug h their New Zealand agent, Chemiplas NZ Limited, supplies three flocculants in solid fo rm as “Magnasol Floc Blocks”. These “Floc Blocks” are designed to be placed in a flow of water that requires flocculant treatment, and to dissolve slowly and contribute flocculant ch emical to the flow. Preliminary trials using these Floc Blocks during the ALPURT earthworks indicated that Floc Blocks may be effective in treating runoff from earthworks si tes. The Floc Blocks break down in flowing water, supplying chemical to the flow that ca using particles to flocculate and settle more rapidly than would have otherwise occurred. This led to the development of a dosing system for the treatment of sediment-laden runoff.The Use of Flocculants and Coagulants to Aid Suspended Sediment Settlement in Earthworks Runoff - Trials, Methodology and Design

2509784 Beca Carter Hollings & Ferner Ltd Page 2 L1:90873-flocculation report - Becas Rev A In addition, a rain-activated system that trigge rs liquid flocculant chemical to be added to flows entering sediment retention ponds was utilised on a number of these ponds. The results of both aspects of the field trials prov ed that the system were highly effective across a range of storm conditions.



### 1 . 2 G r e e n h i t h e T r i a l s
The ALPURT trials indicated that there were po tential advantages in the use of Floc Blocks in comparison to the use of the rainfall activa ted liquid flocculant and coagulant chemicals, particularly on sites with difficult access, small earthworks areas, or where there was a need for short term treatment only. As a re sult, a more comprehensive study to determine the effectiveness of the Floc Blocks was initiated. The objective of this detailed field assessmen t was to establish practical means of using Floc Blocks that achieve the optimal chemical dose required to effect substantial suspended solids removal in the highly variable flow cond itions to be expected from an earthworks site. A residential subdivision development was sele cted at Greenhithe. The Floc Block trial system was designed and installed before ea rthworks commenced, and remained in place during the bulk earthworks and stabilisation period between December 1999 and June 2000. This report presents the results of the trials, which identified significant problems with the delivery of the chemical.The Use of Flocculants and Coagulants to Aid Suspended Sediment Settlement in Earthworks Runoff - Trials, Methodology and Design

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# 2 CHEMICAL FLOCCULANTS AND COAGULANTS

2.1 Introduction Two types of chemicals were considered for init ial bench testing and field trials as follows:



#  Polyelectrolyte flocculants; and


#  Aluminium coagulants, consisting of Alum inium sulphate (Alum) and Polyaluminium chloride, (PAC)). Prior to detailing these types of chemicals however, the background to coagulation and flocculation is outlined.



### 2.2 Coagulation
Colloidal particles are particles that fall into a general category of very fine material that has electrostatic surface charges. In general, most colloidal material has a negative charge. Particles with like charges tend to repel each other, preventing the forming of coagulated particles. These characteristics cause the co lloidal particles to remain in solution. Destabilising colloidal material to allow coagul ation and settlement to occur is achieved by adding reagents that develop positive charges. Positively charged ions in the solution act to destabilise the colloidal matter and allow settl ement of coagulated material to occur as

depicted in Figure 1 1

.

1 Reference: Fernz Chemicals (NZ) Ltd “A Guide to Water and Wastewater Treatment”, undated. Negatively charged clay particles repel each other and rem ain in suspension

Colloi dal particles are attracted to coagulant molecules forming a floc that settles ra pidly

Floc formed around coagulant moleculeThe Use of Flocculants and Coagulants to Aid Suspended Sediment Settlement in Earthworks Runoff - Trials, Methodology and Design

2509784 Beca Carter Hollings & Ferner Ltd Page 4 L1:90873-flocculation report - Becas Rev A Figure 1. Chemical Settling of Colloidal Matter



### 2.3 Flocculation Flocculation occurs after the addition of chemical to destabilise the charges on the colloidal particles in suspension. The particles adhere to each other via the flocculant ions on the surface of the particles. These charged ions provide an opportunity for charged particles in a system to adhere to them, thereby merging indi vidual particles (Figure 2). This results in larger, denser flocs that settle more rapidly.

Figure 2. Colloid Linking Mechanism 2


### 2.4 Polyelectrolyte (Polymer or Polyacrylamide) Flocculants Polyelectrolytes that are potentially suitable for treatment of sediment-laden runoff are available as either concentrated liquids or solids. Three groups are available:



#  Anionic;


#  Cationic; and,


#  Non-ionic. Anionic polyacrylamide is negatively charge d flocculant commonly used for industrial applications including raw potable water clarific ation, and for clarification, thickening and dewatering of wastewater and sludge. Because these polymers have a high affinity for solids, the remaining concentration in treated wa ters is very low in all but serious overdose situations. Cationic polyacrylamides are positively charged and are commonly used in a number of municipal wastewater treatment plants to im prove solids removal during pre-settlement. They are recognised as flocculants with grea ter toxicity implications for fish and other aquatic organisms than anionic or non-ionic poly electrolytes. This is because the gills of fish are negatively charged, and the cationic polymer binds to them resulting in mechanical suffocation

.

2 Reference: Fernz Chemicals (NZ) Ltd “A Guide to Water and Wastewater Treatment”, undatedThe Use of Flocculants and Coagulants to Aid Suspended Sediment Settlement in Earthworks Runoff - Trials, Methodology and Design

2509784 Beca Carter Hollings & Ferner Ltd Page 5 L1:90873-flocculation report - Becas Rev A Long polymer chains of polyacrylamide poly electrolyte can degrade readily in aquatic systems. The oil fraction in liquid polyelec trolyte is readily degradable. Acrylamide, which is a raw material from which polyacryla mide polyelectrolytes are made, is highly toxic, and is reported by suppliers to be pr esent in polyacrylamide polyelectrolytes at a maximum concentration of 0.1%. However, Acrylamide is completely degraded by bacteria in surface waters

. Bench testing indicated that a number of polyacrylamide formulations achieved accelerated settlement of suspended solids fr om runoff from the ALPURT construction works. However, in practise, major difficulties were identified with the use of liquid polyacrylamides for dosing of stormwater on remote sites. These difficulties included: 1. Liquid polyacrylamide concentrations as supplied, are highly viscous and would require onsite pre-dilution with water to achieve a suitable consistency for dosing and mixing with sediment-laden runoff. 2. Pre-dilution would require special mi xing equipment and storage tanks, which would be difficult to provide for on remote sites with difficult or limited access. 3. Pre-dilution would require electric power that would present a difficulty on remote sites with difficult access. 4. After pre-dilution, the polyacry lamide has a limited storage life.


### 5. Complete mixing with stormwater would still be difficult because of the high viscosity of the prediluted polyacrylamide. Three solid polyacrylamide products marketed by Allied Colloids as ‘Floc Blocks’, which act to settle solids in stormwater, were evaluated in bench tests utilising sediment laden runoff from the ALPURT construction works. The products were:



#  Percol AN1 - an anionic polyacrylamide blend;


#  Percol AN2 - an anionic polyacrylamide blend; and


#  Percol CN1 - a cationic polyacrylamide blend.

Bench tests were undertaken to determine the performance of the three Floc Block formulations, using sediment laden runoff from both a clay soil site, and a limestone soil site. The results showed that AN2 performed best for both runoff types. The Floc Blocks used in the trials were 300 x 100 x 85 mm in size and weighed approximately 3 kg.
Because AN2 was the most effective in terms of the dose required to achieve satisfactory reduction of suspended solids, and also be cause anionic polyacrylamides have lower toxicity to aquatic organisms than cationic polyacrylamides, AN2 was selected for ongoing trails. Effective dose rates were between 1 and 4mg/L of dry AN2 (not the partially hydrated Floc Block formulation). Subsequent testing showed that the addition of AN2 above theThe Use of Flocculants and Coagulants to Aid Suspended Sediment Settlement in Earthworks Runoff - Trials, Methodology and Design

2509784 Beca Carter Hollings & Ferner Ltd Page 6 L1:90873-flocculation report - Becas Rev A optimum dose range led to a reduction in flo cculation and suspended sediment removal. Analysis of pH in bench tests showed that AN2 did not affect pH even at excessive dose rates of 6 and 8 mg/L.



### 2 . 5 A l u m i n i u m C o a g u l a n t s

2.5.1 Common Uses Aluminium coagulants are commonly used fo r potable water treatment and removal of sediment from stormwater. In the United Stat es there is considerable experience with the direct dosing of eutrophic lakes with Alum , which has achieved major reductions in suspended solids, phosphorus, nitrogen, biochemical oxygen demanding substances, chlorophyll ‘a’ and turbidity for long periods following dosing. Bench testing showed that both Alum and Polyaluminium Chloride (PAC) achieved encouraging results for the settlement of susp ended solids in sediment-laden runoff from ALPURT.



### 2.5.2 Toxicity and Chemistry in Water The aluminium coagulants contain high conc entrations of the toxic ionic form of aluminium. The USEPA freshwater ambien t water quality criteria for dissolved aluminium at a pH between 6.5 and 9.0 are as follows: Chronic: 0.087mg/L (4 day average not to be exceeded) Acute: 0.750mg/L (1 hour average not to be exceeded) It is generally accepted that dissolved aluminium at a concentration between 0.050 and 0.100mg/L and a pH between 6.5 –and 8.0 presents little threat of toxicity. However, at lower pH, the toxicity increases with an effect of possible major concern being the coagulation of mucus on the gills of fish. After the addition of aluminium coagulants to water containing dissolved and/or suspended matter, dissolved aluminium ions are rapidly incorporated into microscopic aluminium hydroxide and aluminium phosphat e precipitates. As they form these precipitates, they combine with phosphorus, suspended solids, metals, and other dissolved and suspended matter. The insoluble precipitat es that are formed from this process are stable. As the particle size increases, the density also increases, and they tend to sink towards the bottom (as shown in Figure 2). The toxic aluminium derived from the coagulant dose is very rapidly reduced by the precipitation and coagulation reactions. Even if coagulant is added at a dose rate in excess of that required for effective removal of so lids and nutrients, the dissolved aluminium is still reduced very rapidly to a low concentrat ion with no serious toxicity implications.The Use of Flocculants and Coagulants to Aid Suspended Sediment Settlement in Earthworks Runoff - Trials, Methodology and Design

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### 2.5.3 Toxicity of Floc Alum floc is not toxic to benthic organisms. Small planktonic crustaceans present in water samples collected from sediment retention po nds used for coagulation testing during the ALPURT trial program, showed no toxicity du ring the coagulation and settlement periods.



### 2.5.4 Chemistry of Deposited Sludge or Sediment Insoluble precipitates formed by the additi on of Alum to water containing dissolved and/or suspended matter are stable. International research conducted on the toxicity and leaching characteristics of Alum sludge ha s indicated that most pollutants are tightly bound to the aluminium matrix with little or no affinity for release from either dried or wet sludges. The aluminium appears to be tightl y bound in Alum treated sediment under both reduced and oxidised conditions, and at pH ranges between 5 and 7. The release of aluminium from Alum treated sediment appears to be relatively unaffected by changes in the redox potential within the sediment.The Use of Flocculants and Coagulants to Aid Suspended Sediment Settlement in Earthworks Runoff - Trials, Methodology and Design

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# 3 DESIGN OF A LIQUID CHEMICAL DOSING SYSTEM

3.1 Introduction After bench testing had demonstrated that the liquid Alum and PAC flocculant achieved satisfactory suspended solids settlement from construction stormwater samples, a field trial was designed. Initial consideration was given to the installati on of a runoff proportional dosing system of which the major components would be a flow measurement weir or flume; an ultrasonic sensor and signal generating unit, and a battery driven dosing pump. Together with the cost of preparatory site work, chemical stor age tanks and secure housings, the cost per treatment system was estimated to be approximately $12,000. Although the use of a pressure transducer for flow measurement would have reduced the cost to approximately $9,000, difficulties were envisaged with maintenance of the flow measurement weir or flume, because of the high quantity of bedload that can be generated in runoff from earthworks sites. An alternative system, which provides a chem ical dose proportional to rainfall, was developed. The rainfall driven system had the major advantage that it did not require either a runoff flow measurement system or a do sing pump. This system had a total cost of approximately $2,400 per installation.



### 3.2 Design of Rainfall Driven Dosing System

3.2.1 Principles of Operation The main components of the rainfall driven dosing system are shown diagrammatically in the Figures 3 to 7.
The rainfall volume collected from a rainfall ca tchment tray is used to displace the liquid treatment chemical from a storage or reservoi r tank into the stormwater diversion channel prior to entering the sediment retention pond. The size of the constructed catchment tray is determined by the size of the catchment draining to the sediment retention pond. The practise developed through the ALPURT trials has assumed that 100% of the rain fallin g onto saturated earthworks areas, and 60% of the rainfall onto the stabilised contributing catchment area will require treatment at the design dose rate.The Use of Flocculants and Coagulants to Aid Suspended Sediment Settlement in Earthworks Runoff - Trials, Methodology and Design

2509784 Beca Carter Hollings & Ferner Ltd Page 9 L1:90873-flocculation report - Becas Rev A Figure 3. Rainfall Activated Flocculati on System Housing Detail – Side View

Figure 4. Rainfall Activated Flocculation Housing 

### Rainfall tray Pipe to Housing rainwater header

Posts
Front Rear

Lockable Ply door frame Hose to convey flocculant to inflow

Concrete footing Header

Tank

Displacement Tank

Flocculant ReservoirThe Use of Flocculants and Coagulants to Aid Suspended Sediment Settlement in Earthworks Runoff - Trials, Methodology and Design

2509784 Beca Carter Hollings & Ferner Ltd Page 10 L1:90873-flocculation report - Becas Rev A Figure 5. Rainfall Activated Flocculati on System Catchment Tray – Plan View

Rainfall from the catchment tray (Figures 5 and 6) is drained into a header tank (Figure 7). This provides a storage capacity that avoids dosing during initial rainfall following a dry period and to attenuate dosing at the beginning and end of a rainstorm (to simulate the runoff hydrograph).

Figure 6. Rainfall Catchment Tray Housing roof

Lockable door Drain to header t ank Catchment Tray constructed of ply that has been sealed with silicon sealant (or similar) along the joins.
Grade of catchment tray approximately 10:1 (H:V)

50 x 50mm battens fixed to the edges of the trayThe Use of Flocculants and Coagulants to Aid Suspended Sediment Settlement in Earthworks Runoff - Trials, Methodology and Design

2509784 Beca Carter Hollings & Ferner Ltd Page 11 L1:90873-flocculation report - Becas Rev A Figure 7. Rainfall Activated Flocculation Dosing Detail

From the header tank, the rainwater discharges by gravity into a displacement tank which floats in the chemical reservoir (Figure 7). As the displacement tank fills with rainwater, chemical is displaced through the outlet in the reservoir tank and then flows by gravity to the dosing point. Mixing of the flocculant with flows into the pond needs to occur. This can be achieved by discharging the outlet pipe into an area of turbulent flow prior to entering the pond. The outlet pipe is shown in Figure 4. In this instance, the pipe discharges into the sediment laden flow immediately prior to the pond. Howe ver, depending on site characteristics, it may be more appropriate to discharge into flows further up the catchment.



### 3.2.2 Example of Volumetric Design The example given here is for an earthwor ks catchment of 1 hectare and chemicaltreatment using PAC. The required dose is determined by bench tests using sedimentladen runoff from the site, or from a site that has similar soil characteristics.
Calculation of Rainwater Catchment Tray Area The following process is used to calculate the rain catchment tray area:
3⁄4 Determine the dose of aluminium to achieve the optimal coagulation and settlement for the given soil characteristics on site, determine by bench testing
3⁄4 For this example, use 8mg/L Rain in from Catchment Tray

slow start/attenuation Header High rate outlet Tank No dosing volume (25 mm Ø)

Low rate outlet (4 mm Ø)
Flocculant discharged to sediment pond inflow
Displacement Tank

Flocculant Reservoir Tank

N.B. Size of Header, Displacement and Reservoir Tanks dependant on volume of flocculant required to be housed for treatment.The Use of Flocculants and Coagulants to Aid Suspended Sediment Settlement in Earthworks Runoff - Trials, Methodology and Design

2509784 Beca Carter Hollings & Ferner Ltd Page 12 L1:90873-flocculation report - Becas Rev A 3⁄4 Liquid PAC obtained from Fernz Chemicals (NZ) Ltd contains 10.1% of Al 2 O 3 by weight. This is equivalent to 53,470 mg /kg aluminium or 64,164 mg/L aluminium, as the density of PAC is 1.20.
3⁄4 At 8 mg of aluminium per litre, 1 L of PAC would treat 8,020 L of stormwater.
3⁄4 Assume a catchment area of 1 ha and th en assume runoff to be treated is the equivalent of 50mm of runoff
3⁄4 For a 1 ha catchment, this is 500 m 3 of stormwater runoff
3⁄4 The volume of PAC required to treat 500 m 3 of runoff at 8 mg Aluminium /L is 62.3 L.
3⁄4 The density of PAC is 1.2. Therefore, to displace 62.3 L of PAC, it would require a volume of 74.8 L of rainwater to be collected.
3⁄4 To collect 74.8 L of rainwater from a 50mm rainfall event, it would require an area of 1.5 m 2

. Table 1 presents the rainfall catchment required for different PAC dose rates. The optimal does rate should be determined by bench testing for each site, or as per that applied to a site with similar soil characteristics.
Table 1. Rainfall Catchment Tray Area Required for Different PAC* Dose Rates.
Aluminium Dose Required (mg/L) Catchment Tray Area/Hectare (m 2 ) Ê 2 0.375 4 0.75 6 1.125 8 1.5 10 1.875 12 2.25
Note: * PAC with 10.1% of Al 2 O 3 by weight

Ê Stabilised catchment calculated at 60% of area given.

Header Tank The header tank provides:
3⁄4 Zero chemical discharge until a pre-selected quantity of rain has fallen, to allow for initial infiltration and saturation of dry ground before runoff commences;
3⁄4 A slow start to the dosing rate to allow for the response time of runoff flowing off the site at the beginning of a storm; and
3⁄4 An extension of the dosing period beyond the rainfall period to provide treatment of runoff that occurs following cessation of rainfall.The Use of Flocculants and Coagulants to Aid Suspended Sediment Settlement in Earthworks Runoff - Trials, Methodology and Design

2509784 Beca Carter Hollings & Ferner Ltd Page 13 L1:90873-flocculation report - Becas Rev A The zero discharge rainfall can be adjusted for site characteristics, and the slow start/attenuation characteristics can also be regulated for site-specific conditions by changing the discharge rate from the low rate header tank outlet, and by providing low rate outlets at different levels from the head er tank. The standard header tank design adopted for ALPURT provides for up to 10mm of rainfall before dosing commences. This would require provision of a delayed start volume below the low rate outlet of the header tank of 10 L/m 2 of rainfall catchment tray. The ALPURT header tank design for small ea rthworks catchments has a low rate outlet consisting of a 4mm internal diameter hose, and a high rate outlet that has sufficient capacity to carry the maximum predicted flow from short-term rainfall of about 40mm/hour. For catchments with long flow paths, or other site conditions that attenuate the flow to the treatment site and retention pond, the capacity of the low rate outlet from the header tank can be further reduced. Additional low capacity outlets installed at different levels within the header tank live storage volume, enable th e dose rates to more accurately follow the runoff hydrographs.
Displacement Tank Size The displacement tank (Figure 7) is to be a neat fit inside the reservoir tank. A larger displacement tank and reservoir tank system w ill reduce the degree of servicing required. The minimum displacement tank capacity should be the 24-hour rainfall for a 2-year return period. In the ALPURT area this is approx imately 86 mm of rain, giving a volume of 129 L from a constructed catchment of 1.5 m 2

. A standard design was adopted for the ALPURT project. This consisted of a 400 L displacement tank within a 550 L reservoir tank, which provides for the dosing of up to 320 L of PAC.

Chemical Reservoir Tank The chemical reservoir tank (Figure 7) should have sufficient capacity to provide for the dosing of runoff from a major storm, based on an alysis of local rainfall statistics (refer to displacement tank sizing above). The outlet of the ALPURT standard design system was located at a level that is reached by filling with 2 x 200 L drums of PAC.The Use of Flocculants and Coagulants to Aid Suspended Sediment Settlement in Earthworks Runoff - Trials, Methodology and Design

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### 3.3 Set Up and Servicing of the Rainfall Driven Dosing System

3.3.1 Header Tank
The water level of the header tank is set to allow for a certain rainfall before chemical dosing starts. In summer, after a week or more without rain, this was found to be approximately 10 mm in the ALPURT area. It was noted on one occasion during the summer, when approximately 10mm of rain fell in 15 minutes on dry ground, substantial runoff occu rred, delaying the start of dosing. This lead to insufficient dosing of the first flush into the pond. It is noted however, that there was some buffering effect of these flows due to the pond containing only clean water prior to the storm. In wet weather, the header tank is set with no delay in dosing. When a dry period occurs following rain, the header tank volume below the low capacity outlet is reduced to provide no dosing of the first part of the next storm ev ent. This is to prevent overdosing of a pond, with possible implications in terms of reduced pH, an increased free aluminium concentration, and also conserved PAC. Wa ter may be removed from the header tank using a siphon.



### 3.3.2 Refilling with Chemical When dosing has occurred and the capacity of the dosing system is reduced to the degree that there is insufficient to dose a major stor m, the displacement tank has to be emptied and the chemical reservoir refilled. The displacement tank may either be emptied using a siphon, or baled out by hand. The chemical reservoir can be filled using a drum pump, to pump from a 200 L drum.



### 3.3.3 Monitoring and Adjustment for Changing Site Conditions Each new chemical treatment system is moni tored carefully during the first few rainfall events to check that the system is effective, an d to ensure that overdosing is not occurring. If overdosing is suspected because the pond dead storage water is exceptionally clear, samples are taken for pH and dissolved aluminium analysis. If overdosing occurs or it is clear that the quality of stormwater runoff is improving because of stabilisation of the site, the chemical dose may be reduced by reducing the size of the catchment tray. This may be done by placing a diagonal batten across the tray with a hole through the tray rim at the lower corner, so that water from the tray area above the batten discharges to waste.The Use of Flocculants and Coagulants to Aid Suspended Sediment Settlement in Earthworks Runoff - Trials, Methodology and Design

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# 4 THE ALPURT TRIALS

4.1 Introduction
A trial pond was established at Bawden Rd (nor th of Albany) where significant cut and fill earthworks were taking place in the catchm ent. The works took place in limestone (Onerahi chaos) soils. Two trial ponds were constructed in series with a valved gravity connection between the ponds and valved outlets. The chemical treatment trials using Alum were carried out at an aluminium dose rate of 5.5 mg/L. The treatment system was monitored ca refully to determine effects of the Alum dosing on suspended solids removal, pH and the dissolved aluminium concentration in the discharge of the retention pond. The Alum system performed satisfactorily in terms of reduction of suspended solids under a range of rainfall conditions varying from very light rain to a very high intensity, short duration storm, where 24mm of rain fell over a period of 25 minutes. The pH of the stormwater runoff at the Bawden Road trial pond site was between 7.9 and 8.2. Dosing with Alum at 5.5 mg/L reduced the pH by approximately 0.5 pH units. Dissolved aluminium concentration in the discharge from the retention pond averaged approximately 0.10 mg/L, and was often lower than the dissolved aluminium concentration in the inflow to the pond from the site.
Bench tests were undertaken to evaluate whether an excessive dose of Alum would result in increased levels of dissolved aluminium concentrations. The results (Table 2) showed that the addition of aluminium at up to 12.6 mg/L did not result in any increase in the dissolved aluminium concentration in the reacted sample, after a one-hour settlement period. Because of the high natural pH of limes tone soil runoff, the addition of Alum up to 12.6mg/L aluminium did not result in a significant reduction in pH.
The treatment benefit of Alum addition was mo st evident for the intense storm conditions for which the runoff flow exceeded the hydraulic capacity of the retention pond (i.e. the [pond discharged via the spillway). With Alum treatment, the suspended solids removal was 92% compared with removal without Alum treatment of about 10% for a similar storm on the same catchment with the same retention pond.The Use of Flocculants and Coagulants to Aid Suspended Sediment Settlement in Earthworks Runoff - Trials, Methodology and Design

2509784 Beca Carter Hollings & Ferner Ltd Page 16 L1:90873-flocculation report - Becas Rev A Table 2. Results of Alum Addition to Limestone Soil Runoff to
Evaluate Effects of Alum Overdose

Note (1) fine sediment/flocs noted in filtered samples


### 4.2 Polyaluminium Chloride (PAC) Trial

4.2.1 Advantages of PAC It became clear that there was a need during the ALPURT project for the chemical treatment of runoff from clay soil in catchments that naturally produce more acidic runoff. It was determined to investigate the use of PA C as a coagulant in this instance, as PAC is less acidic than Alum. Table 3 presents comparative data for Alum and PAC treatment of representative runoff samples from clay soil catchments.
Table 3. pH Data for Alum and PAC Treated Stormwater Samples Source Coagulant Al conc. (mg/L) pH Alkalinity ) SS (mg/)L Oteha Valley Rd Nil (raw) - 5.64 1 1504 SE Pond Alum 8 4.42 <1 71 Alum 12 4.34 <1 71 PAC 8 4.64 <1 107 PAC 12 4.63 <1 85 Lonely Track Rd Nil (raw) - 6.68 16 680 Gully1 Alum 8 4.64 <1 117 Alum 12 4.54 <1 113 PAC 8 6.03 7 81 PAC 12 5.54 3 112 Awanohi Rd Nil (raw) - 7.15 60 1130 Adj. Okura Rd Alum 8 5.88 13 84 Alum 12 4.85 <1 84 PAC 8 6.71 43 229 PAC 12 6.45 35 78 Sample Al (mg/L) Settlement time (hr) pH SS (mg/L) Diss. Al (mg/L) A 0 1 7.92 2760 1.1 A 1 1 8.12 2560 0.11 A 2.1 1 7.95 1770 0.68 (1)
A 4.2 1 7.86 445 0.15 A 6.3 1 7.78 267 0.43 (1)
B 0 0 8.04 8080 0.075 B 0 1 8.06 3690 0.071 B 4.2 1 7.73 728 0.50 (1)
B 8.4 1 7.69 189 0.19 B 12.6 1 7.5 90 0.13The Use of Flocculants and Coagulants to Aid Suspended Sediment Settlement in Earthworks Runoff - Trials, Methodology and Design

2509784 Beca Carter Hollings & Ferner Ltd Page 17 L1:90873-flocculation report - Becas Rev A Samples were taken after 1 hour of settlement. The alkalinity was measured given the quantity of CaCO 3 (g/m 3 ) in solution. Longer settlement would result in further reduction in suspended solids. The data in Table 3 show s that PAC has a consistently lower effect in terms of pH reduction on receiving waters into which the retention ponds discharged.



### 4.2.2 Results of PAC Trials
The use of PAC for treatment of sediment la den runoff was investigated extensively along the ALPURT project during the 1998/99 summer, an d during the winter of 1999. A total of 21 systems were used, with contributing catc hments ranging between 0.5 and 15 hectares. The overall treatment efficiency of PAC trea ted ponds in terms of suspended sediment retention was between 90 - 99% for ponds with a good physical design. Lower treatment efficiency occurred where the ponds did not operate properly due to mechanical problems with decanting devices, or physical problems such as multiple inflow points, high inflow energy, or poor separation of inlets and outlets. Table 4 presents representative data for PAC dosed stormwater from clay soils earthworks catchments, and shows that a high degree of suspended solids reduction is achieved in PAC dosed ponds.
Table 4. Suspended Solids Removal from PAC Treated Stormwater Pond (a) Date Inflow Flow SS Outflow Flow SS Reduction of SS (%)

Mason’s Rd 28.11.1998 3 26 300 3 144 99.4 Mason’s Rd 04.12.1998 2 5 100 2 40 99.2 OVR E 13.06.1999 15 1 639 8 51 96 OVR E 04.07.1999 2 749 2 56 92 23800E 28.11.1998 8 14 800 6 966 93 23800E 22.01.1999 1 18 700 2 67 99 B1 Gully 08.04.1999 0.3 4 300 0.4 3 99.9 B1 Gully 01.05.1999 0.5 16 900 3.0 59 99.6

The data in Table 5 indicates considerable variation of inflow suspended solids concentration between the ponds sampled. These significant variations reflect the condition of the catchments and the nature of current works occurring. All the treated ponds achieved good suspended solids removal (77 - 98%) compared to that of untreated ponds (4 - 12%). PAC dosing caused an ob vious reduction in pH in all ponds except Lonely Culvert.The Use of Flocculants and Coagulants to Aid Suspended Sediment Settlement in Earthworks Runoff - Trials, Methodology and Design

2509784 Beca Carter Hollings & Ferner Ltd Page 18 L1:90873-flocculation report - Becas Rev A Tables 5. Inflows and Discharges of PAC Treated Ponds Inflow Outflow Pond Time

Flow L/s SS mg/L pH Al mg/L Flow L/s SS mg/ L pH Al mg/L Reduction of SS (%)

0850 12 238 8.97 0.084 9 53 6.66 0.026 77 1.3.8 Over 1135 20 253 9.97 0.077 12 55 6.79 0.068 78 0938 40 25830 6.83 0.052 3 266 7.62 0.072 98 Lonely 1045 15 13310 6.62 0.093 20 214 7.02 0.018 98 0918 8 399 8.78 0.25 3 40 6.56 0.016 89 21340 1110 7 2564 7.03 0.11 15 57 6.55 0.01 88 0910 6 2132 6.81 0.16 4 65 5.96 0.025 96 D5 1110 7 2564 7.03 0.11 4 56 5.47 0.01 97 1930 12 1571 7.88 0.22 4 1378 7.74 0.31 12 Untreated Pond 1100 9 1522 8.02 0.17 4 1459 7.83 0.29 4

The dissolved aluminium concentrations in the outflows from chemically treated ponds varied between 0.010 - 0.072 mg/L, and were lower in most of the outflows compared with the inflows. The dissolved aluminium concentration in the outflow from the untreated pond was much higher (0.29 - 0.31 mg/L) than in the outflows from the treated ponds.
It has been noted that the dissolved aluminium concentration is related to the characteristics of the suspended solids, with high concentrations of dissolved aluminium occurring in samples that also had high concentr ations of very fine suspended solids. This trend has been discussed with a chemical analyst, and is attributed in part to small quantities of very fine solids passing through the filter that is used to filter samples before analysis. Dissolved aluminium by definition, is the fractions passing through the 0.45- micron filter. If very small sediment particle s pass through the filter, they also contribute to the dissolved aluminium concentration.
The dissolved aluminium concentrations in the outflows from the treated pond samples recorded in Table 6 were below the USEPA chronic criterion of 0.087 mg/L (4 day average not to be exceeded), and well below the acute criterion of 0.750 mg/L (1 hour average not to be exceeded). The data in Table 6 indicates significant remo val of suspended solids, particularly in the ponds with high-suspended solids in the inflows. In contrast, the untreated pond had the highest concentration of suspended solids concentration in the outflow. The data for the Mason's Rd pond provides an example of a PAC overdose situation, in which the pH after dosing had been reduced to 4.44, and the dissolved aluminium concentration was at a high level of 1.lmg/L. The outflow data for pond 2444OW also indicates a possible PAC overdose, with a low pH of 4.70, although the dissolved aluminium was not markedly elevated.The Use of Flocculants and Coagulants to Aid Suspended Sediment Settlement in Earthworks Runoff - Trials, Methodology and Design

2509784 Beca Carter Hollings & Ferner Ltd Page 19 L1:90873-flocculation report - Becas Rev A Table 6 PAC Dosed Sediment Retention Pond Monitoring Data, 21.10.99 Pond Inlet/ Outlet Time Flow (L/S) pH SS (mg/L) Al (mg/L) Hard (mg/L) Reduction of SS (%) Masons In 1700 3 6.44 4704 0.02 72 Out 1705 3 4.44 41 1.10 49 99 OVRE In 1720 12 8.80 23240 0.29 65 Out 1725 10 9.04 272 0.07 95 98 OVRW In 1740 8 6.86 28845 0.02 194 Out 1745 10 6.89 338 0.02 85 98 2444OW In 1750 3 - 164 0.20 58 Out 1745 2 4.70 15 0.34 47 90 D5 In 1815 6 7.65 770 0.03 206 Out 1820 5 6.15 36 0.01 159 95 2134OW In 1825 3 10.73 128 0.31 64 Out 1827 4 6.84 14 0.03 81 89 Debs In 1845 4 11.47 752 0.21 135 Out 1850 6 9.82 279 0.31 98 62 Lonely In 1855 4 11.12 254 0.07 113 Out 1900 8 8.31 72 0.16 113 71 Untreated Out 1835 3 8.63 712 0.06 89
The Table 6 data clearly shows the effects of lime stabilisation on the road foundation, on the pH of the inflows to ponds 2134OW, Debs, and Lonely. The relatively high dissolved aluminium concentrations in the discharges fr om Deb's and Lonely appeared to be related to the very fine suspended solids in the discha rges. Hardness data was obtained as part of ongoing investigations of the chemistry of PAC dosing of runoff from clay soils, with relatively low pH. Table 7 provides additional data on pH, suspended solids, and dissolved aluminium in discharges of PAC treated ponds.

Table 7. Discharge Data from PAC Dosed Sediment Retention Ponds

Pond pH SS (mg/L) Diss. Al (mg/L) Masons Rd 4.68 10 1.50 OVRE 6.20 22 0.07 Lucas E 5.25 67 0.07 24100E 7.19 54 0.03 Kewa Rd 5.26 16 0.06 D5 6.19 46 0.12 21340W 6.90 10 0.05 21900E 7.46 258 0.10 Deb’s culvert 8.01 94 0.24 Lonely culvert 7.21 63 0.08 Untreated Pond 7.45 438 0.12The Use of Flocculants and Coagulants to Aid Suspended Sediment Settlement in Earthworks Runoff - Trials, Methodology and Design

2509784 Beca Carter Hollings & Ferner Ltd Page 20 L1:90873-flocculation report - Becas Rev A The data in Table 7 for Mason's Rd illustrates a PAC overdose situation with low pH and high dissolved aluminium. The concentrations of dissolved aluminium were generally low, even for the discharges from Lucas East and Kewa Rd ponds that had relatively low pH. The Deb's culvert pond data in Table 7 shows the influence of lime stabilisation in the catchment. The pond discharge also ha d a relatively high dissolved aluminium concentration. The data presented in Tables 4 to 7 show that PAC treatment of clay soils runoff can achieve a very high degree of suspended solids removal without significant adverse effects on the chemistry of the treated runoff. Howeve r it is necessary, to avoid overdosing, which can reduce the pH and can then increase the dissolved aluminium concentration in the discharge. The runoff from clay soils that are naturally acidic requires particular care to ensure that overdosing does not occur. The data obtained for clay soils runoff indicates that the dissolved aluminium concentration in the treated runoff does not in crease rapidly until the pH is reduced below 5. At this stage it is desirable to avoid discharges having a pH below 5.5.The Use of Flocculants and Coagulants to Aid Suspended Sediment Settlement in Earthworks Runoff - Trials, Methodology and Design

2509784 Beca Carter Hollings & Ferner Ltd Page 21 L1:90873-flocculation report - Becas Rev A 

# 5 PRELIMINARY FLOC BLOCK TRIALS

5.1 ALPURT Floc Block Trial

5.1.1 Background Preliminary field trials were undertaken as part of the ALPURT motorway project using the AN2 Floc Blocks to treat sediment-laden runoff from limestone-derived soils (Onerahi Chaos). The first trials were undertaken by placing the Floc Blocks in plastic mesh bags (" netlon" ) in plywood flumes through which the runoff from the site was directed (refer to Figure 8).

Figure 8: Floc Blocks and Flume Detail Those trials encountered problems with the high bedload of granular material in the runoff flow. The material accumulated against, part ially buried, and stuck onto the Floc Blocks inhibiting solubility of the chemical.
The trial was then moved to a channel that was formed to divert flows following some primary treatment within in a forebay to the sediment retention pond (Figure 9). The reconfigured array of Floc Blocks achieved good treatment for low flows of estimated at 2 L/s new Floc Block when the concentration of suspended solids contained within the storm runoff was between 10,000 to 20,000 mg/L, and significant bedload from the inflows had occurred. The quantity of solids in th e runoff, following an intense rainfall event continued to be a problem, as both the forebay and Floc Block channels were filled with sediment (Figure 10).The Use of Flocculants and Coagulants to Aid Suspended Sediment Settlement in Earthworks Runoff - Trials, Methodology and Design

2509784 Beca Carter Hollings & Ferner Ltd Page 22 L1:90873-flocculation report - Becas Rev A Figure 9: Floc Blocks within Channel between Forebay and Pond

Figure 10: Pond Inlet Channel Full of Sediment

As the earthworks area was gradually stabilis ed, the quality of runoff to the preliminary trial site was improved. Additional tests in a new flume also showed that effective treatment was achieved at flows up to 2 L/s per new Floc Block for runoff with suspended solids concentrations up to 5,000 mg/L.



### 5.1.2 Results Preliminary findings were as follows:



#  A constant stormwater flow through a Floc Block treatment flume is best in terms of providing the optimum chemical dose for susp ended solids removal. It is difficult toThe Use of Flocculants and Coagulants to Aid Suspended Sediment Settlement in Earthworks Runoff - Trials, Methodology and Design

2509784 Beca Carter Hollings & Ferner Ltd Page 23 L1:90873-flocculation report - Becas Rev A provide an array of Floc Blocks that provide optimal dosing for a highly variable stormwater flow.


#  The performance of a Floc Block treatment system is dependent on both flow and suspended solids concentrations.


#  It is desirable to restrict the maximum fl ow through the treatment flume for any Floc Block system. A practical flow limit is appr oximately 20 L/s. For this reason, either multiple flumes should be used, or the catchment discharging flows minimised to the extent that flows in excess of 20 L/s are not likely. Alternatively, larger flows could be bypassed via a diversion mechanism (e.g. th rottling the flows prior to discharging through the flume).


#  The treatment capacity of Floc Block AN2 on limestone derived soil (Onerahi chaos) runoff proved to be approximately 2 L/s per block at 10,000 mg/L suspended solids, and approximately 1 L/s per block at 20,000 mg/L suspended solids.


#  Floc Block treated stormwater requires le ss time for settlement of suspended solids than aluminium coagulant treated stormwater because the flocs that form are large, dense, and fast settling. Therefore, it is considered that retention ponds utilising a Floc Block system do not need to be as large as those for an aluminium coagulant treatment system.


#  Floc Block treatment has a high potentia l for removal of suspended solids from stormwater with consistent quality, part icularly for small catchments; when flow balancing can be achieved prior to treatment.



### 5.2 Greenhithe Floc Block Trial

5.2.1 Background Despite the limited research undertaken, the preliminary results of the trial using Floc Block during the ALPURT development were considered to be encouraging enough to conduct a specific trial. The trial aimed to: (1) Determine the effectiveness of the Fl oc Block form of polyacrylamide; and (2) Design of a robust dosing system for de livery of the chemical such that the system could be rapidly and easily transferred to other sites.
The trial site selected consisted of a four-hecta re area of pastoral farmland with an easy contour falling from south to north towards a ti dal creek, which is a tributary of the Lucas Creek Estuary. The earthworks design for the site required the stripping of topsoil from the entire site, and bulk earthworks for cut and fill balance within the site. During the earthworks stage, the site was di vided into three stormwater catchments, each of which drained to a sediment pond with dead storage, live storage, and a floating decant, operating within the live storage range.The Use of Flocculants and Coagulants to Aid Suspended Sediment Settlement in Earthworks Runoff - Trials, Methodology and Design

2509784 Beca Carter Hollings & Ferner Ltd Page 24 L1:90873-flocculation report - Becas Rev A The trial site was the sediment pond for th e largest earthworks catchment that varied between 2 - 3 ha at different stages of the earthworks. The sediment-laden runoff was collected by cut-off bunds, which combined to form a single channel about 15m before entering the sediment retention pond. The trial was undertaken in a purposely-built flume, located in the sediment-laden runoff flow path immediately before the sediment pond (Figures 8 and 9).



### 5.2.2 Flume Design The preliminary trials had shown the importance of achieving a moderate degree of turbulence in the Floc Block area to assist the chemical in dissolving into the flow, and had indicated that there were potential difficult ies with achieving the optimum dose for a highly variable stormwater flow regime. The design of the trial flume provided cages, which housed the Floc Blocks vertically. They could be slid in and out of the flume to enable the number of cages in a flume to be varied. The flume was roofed both to inhibit interference, and to shade the Floc Blocks to minimise the tendency to break down when exposed to sun and air (Figure 10). The cages were designed to provide vertical slot flow paths around the Block (Figure 9), with the velocity of the water, and the interference from the mesh and the Floc Block providing turbulence. The cross section of the flow path could be reduced by placing pieces of plywood against the side of the ca ge. This system provided for increasing exposure of the Floc Block surface to the stor mwater as the flow and depth of stormwater increased. The flume contained four 200mm wide by 400mm deep channels, installed on a slope of about 1 in 10. The flume was sealed into a bund, with provision for very high flows to go over the top of the flume (Figure 8). Two 100mm deep weirs were initially placed across the upstream ends of two of the flumes to better direct low flows (Figure 9). A forebay of approximately 10m 3 was required up slope of the flume to trap bedload and heavy material that could interfere with Floc Block operation. Because of site constraints, the contractor provided a forebay of some 5m 3

.



### 5.2.3 Trial Results

Initial Period Without Rain The Floc Blocks were installed on 17 December 1999. On 24 December 1999, when no stormwater had passed through the flumes, it was noted that there was some cracking of the Floc Blocks as a result of drying. On 4 January 2000, there had been no stormwater through the flumes, and the breakdown of Floc Blocks was quite serious (as shown in Figure 10). The breakdown of the Floc Blocks is a potential problem. It may result in the transport of pieces of unreacted Floc Block into the sediment pond by stormwater flow after a dryThe Use of Flocculants and Coagulants to Aid Suspended Sediment Settlement in Earthworks Runoff - Trials, Methodology and Design

2509784 Beca Carter Hollings & Ferner Ltd Page 25 L1:90873-flocculation report - Becas Rev A period. The larger pieces that fell off the Blocks swelled when exposed to stormwater and formed a sticky mass that blocked the bottom of the cage, and thereby interfered with the flow path around the Blocks. Spraying water to the Blocks during dry period s could prevent this problem. It has also been noted that Blocks that have been in wate r for a long period become partially hydrated and when dried are much more resistant to cracking than new Blocks. It is considered that soaking new Blocks before installa tion may prevent major breakdowns.
First Stormwater Flow through Flume The first stormwater flow through the flume was from a very intensive rainstorm on 19 January 2000, when 25-30mm of rain fell over a 30 - 40 minutes. The topsoil had been stripped from the entire site, but bulk of earthworks had not been started. There were two Floc Blocks were in each of the four flumes. Stormwater runoff on 19 January 2000 transported a large bedload off the site to the treatment pond. The forebay and the up-channe l of the flume were completely filled with soil, and the treatment flume was completely clogged (as shown in Figures 11 and 12). About 60m 3 of soil and sediment was retained on the bottom of the pond (measured when pond dried out in February). Pieces of clay of up to 50mm diameter had been transported into the flume. Before the storm on 19 January 2000, the large pieces of Floc Blocks had broken down further than shown in Figure 10. There was a sticky mass of soil and Floc Block in each Floc Block cage after the rain. During the recovery of the Floc Blocks from the sediment, it was noted that the Blocks that had been buried in the sediment, and had been substantially weakened by the penetration of water into the Blocks. The fragments did not have the viscous outer layer that forms when the Blocks are submerged in water. The large pieces of Blocks crumbled very easily, and required careful handing until they were later exposed to water and became viscous on the outside. Data for water samples taken fr om the discharges of the trial pond and the two other site ponds on 20 January 2000, about 24 hours after the rainfall were as follows:
Table 8 Floc Block Trial Discharge Data, 20/01/00 Pond Flow (L/s) PH SS (mg/L) Trial Pond 2.0 4.96 2423 East Pond 0.05 4.47 7252 Upper Pond 0.01 4.68 7310

Low pH values, and the high suspended solids co ncentrations show very poor settlement. Although the Floc Block treatment system wa s overwhelmed by bedload, the trial pond had a lower SS than the other two ponds.The Use of Flocculants and Coagulants to Aid Suspended Sediment Settlement in Earthworks Runoff - Trials, Methodology and Design

2509784 Beca Carter Hollings & Ferner Ltd Page 26 L1:90873-flocculation report - Becas Rev A Very low pH of stormwater runoff from expo sed clay soils below the topsoil layer, has been noted in the North Shore area.



### 5.2.4 Results of Additional Trials
January 2000 A trial was carried out during light rain on 25 January 2000 when the flow through the low flow flumes was about 1 l /s. Good flocculation was achieved with 2 vertical Floc Blocks in a flow of about 0.5 l /s, and there was no indication of overdosing. The pond water levels reached about 150mm below the minimum decant level. The data contained in Table 9 was collected from samples taken on 27 January 2000 from the outlet areas of the three ponds, when th e pond water levels were below the minimum decant levels. There had been no further discharge from the ponds since 20 January 2000.
Table 9 Floc Block Trial Discharge Data, 27/01/00 Pond Flow (L/s) pH SS (mg/L) Trail Pond 0 5.54 510 East Pond 0 4.99 1673 Upper Pond 0 5.05 2948
The data shows the low pH of the stormwat er runoff and the high remaining suspended solids after a settlement period of about 7 days. The suspended solids in the trial pond were; however, substantially lower than in the other two ponds.
February and March 2000. At the beginning of February, Floc Blocks in mesh bags were placed in two flumes (one in a low flow and one in a high flow flume), with cages in the other two. The forebay above the trial flume, was removed during the course of construction of a sewer line, and was not replaced. On 15 March 2000, the lower 20% of the catchment of the trial pond had been topsoiled, and road formation had started. Runoff of high sediment loadings was still considered likely, if intense rain occurred. There was no significant stormwater flow th rough the trial flume during February and March.
April 2000. At the beginning of April, 20% of the trial po nd catchment that had been topsoiled, had a good grass strike. No additional topsoiling had occurred. The remainder was vulnerable to erosion by heavy rain. Completion of some of the permanent stormwater drainage diverted some of the surface runoff from the si te into the culvert discharging to the trial pond (Figure 13).The Use of Flocculants and Coagulants to Aid Suspended Sediment Settlement in Earthworks Runoff - Trials, Methodology and Design

2509784 Beca Carter Hollings & Ferner Ltd Page 27 L1:90873-flocculation report - Becas Rev A Heavy rain fell on 21 April 2000 (total of about 50mm) and stopped on 22 April 2000. Major erosion of the surface stormwater cha nnels above the trial flume occurred, and a topsoil and clay bedload again entered the flum e, which clogged and partially buried cages and the baskets holding the Floc Blocks. The mesh on the Floc Block cages was clogged by fibrous plant material and new grass (Figure 14). Data for the discharge flow from the trial pond on 22 April 2000 when the water level had fallen to close to the minimum decant level is contained Table 10.
Table 10 Floc Block Trial Discharge Data, 22/04/00 Time Flow (L/s) pH SS (mg/L) 1055 0.3 6.98 492

This data shows a major increase in pH compar ed with the first stormwater runoff from the site, and a reduced suspended solids concentration. During the 21/22 April 2000 rainstorm, it was clea r that a substantial quantity of sediment had been discharged into the trial pond via the stormwater system. That part of the inflow to the pond was not chemically treated. About 16mm of rain fell overnight on 24/25 Apr il 2000 and again resulted in erosion of the stormwater channel leading to the flume, and deposition of soil both in front of and through the flume. The flumes with the bask ets remained relatively clean (Figure 5) but the mesh on the cages was again clogged with fibrous material as shown in Figure 15. The stormwater system had again discharged a large quantity of sediment into the trial pond. The data for the discharge flows on 25 April 2000 are contained in Table 11.
Table 11 Floc Block Trial Discharge Data, 25/04/00 Pond Time Flow (1/s) pH SS (mg/1) Trial Pond 0900 0.4 6.78 519 Upper Pond 0930 0.2 5.55 1179

This data shows a marked difference in qualit y between the trial pond and the upper pond samples. The trial pond pH was much higher and suspended solids much lower. The pH difference is attributed to the partial topsoilin g and grassing of the trial pond catchment, while the difference in suspended solids could be partly due to the treatment of some of the inflow to the trial pond.
May 2000 By 08 May 2000 most of the trial pond catchm ent had been topsoiled and new grass was emerging. Rain of about 20mm on 07 – 08 May 2000 produced a stormwater flow which passed successfully through the two low flow flumes with mesh baskets.The Use of Flocculants and Coagulants to Aid Suspended Sediment Settlement in Earthworks Runoff - Trials, Methodology and Design

2509784 Beca Carter Hollings & Ferner Ltd Page 28 L1:90873-flocculation report - Becas Rev A There was moderate rain overnight on 11 – 12 May 2000, easing to stop at 09.00. The live storage volume of the trial pond was full at 08.00 – 08.30, with an overflow of about 5l/s and overflow was reduced completely by 09.35. The flow through the treatment flume was only a minor proportion of the total flow into the trial pond. Most of the inflow was entering via the permanent stormwater reticulation. The culvert outlet of the permanent stormwater reticulation was submerged when the pond live storage was full, shown in Figure 13. Two Floc Blocks were placed in a cesspit that was collecting stormwater from a clay soil area in the upper part of the trial pond catchment at 0830, and three Floc Blocks were placed in the mouth of the submerged culv ert inlet to the trial pond (Figure 13). With two Floc Blocks in mesh baskets, suspended material contained in flume flow was coagulated of about 2.5 L/sec per flume. Th e degree of coagulation was further improved with the addition of two more Floc Block baskets. Build up of soil around the Floc Block cages and some around the Floc Block baskets had occurred.
Data contained in Table 12 was obtained for wa ter quality samples at the trial pond, at the end of the conclusion of the storm event on 12 May 2000.
Table 12 Floc Block Trial Discharge Data, 12/05//00 Time Sample Type Flow (1/s) PH SS (mg/1) 0840 Inflow to flume 5 6.04 1150 0850 Pond Discharge 20 6.61 1870 0900 Inflow via culvert 10 6.97 1980 0935 Pond discharge 10 6.07 1810 1035 Pond discharge 6 6.78 1720

These data show that high concentrations of suspended solids were present in the pond discharge before and after the storm. The Floc Block did not appear to have had any significant treatment effect during the period of peak runoff flow. On 15 May 2000, the Floc Blocks that had been placed in the cesspit on 12 May 2000 were removed as they had become soft and viscous as a result of remaining submerged for a prolonged period. The three Floc Blocks in the culvert exit were left in place, as they were out of the water when the live storage volume of the trial pond had been discharged. They were not as soft as those in the cesspit. On 30 May 2000 (Table 13), samples were taken during a wet period that had been preceded by local, very heavy showers. At th e time of sampling, the water level of the trial pond was about 300mm below the overflow level indicating a heavy shower in the area shortly before samples were gathered.The Use of Flocculants and Coagulants to Aid Suspended Sediment Settlement in Earthworks Runoff - Trials, Methodology and Design

2509784 Beca Carter Hollings & Ferner Ltd Page 29 L1:90873-flocculation report - Becas Rev A Table 13 Floc Block Trial Pond Discharge Data, 30/05/00 Sample Type Flow (L/s) PH SS (mg/L) Inflow via culvert 25 7.09 765

Inflow to flume 5 6.56 331 Pond discharge 8 7.05 494

The data shows that the inflow via the culvert was the major inflow, and had a poorer quality than the inflow through the flume, which consisted of stormwater from a largely stabilised catchment area. The pH of the inflows was higher than that previously recorded, indicating the changing nature of the surfac e soils (more topsoil and grass). Suspended solids concentrations were also lower than previously recorded indicating increased stabilisation of the catchment.

June 2000

. On 06 June 2000, more bedload had been discharged through the flume. The pond water was still highly turbid, and two more Floc Blocks were added to the culvert outlet. On 27 June 2000 during a period of light rain , when the flow through each of the trial flumes was about 1 L/s, significant coagulation occurred with four Floc Blocks in mesh baskets per flume. The pond water level was cl ose to the minimum decant level, and slight floc formation was slightly occurring at th e mouth of the culvert where the flow was estimated to be about 2 L/s. On 29 June 2000, it was evident that approximately 80mm of rain had fallen overnight. The pond surface had overtopped the live storage level, and at 08.30 was estimated to be 250mm below the top of the live storage. Significant erosion had occurred within the channel above the flume. Bedload material was deposited in front of the flume, and in one of the flume channels that still had Floc Block cages. However, shortly after this, good coagulation of the discharge of about 0.3 L/s through each trial flume was observed to be occurring (4 Floc Blocks in mesh baskets per flume), and very slight coagulation was evident in the culvert area (5 Floc Blocks in mesh baskets submerged in flooded culvert outlet). The relatively poor coagulation at th e mouth of the culvert was attributed to the low flow velocity of water passing the Floc Bloc ks. The results of the samples from the trial pond and upper pond discharges on 29.06.00 are contained in Table 14.
Table 14 Floc Block Trial Discharge Data, 29/06/00 Pond Sample type Flow (L/s) pH SS (mg/L) Trial pond Discharge 4 6.96 620 Upper pond Discharge 1.5 5.87 2100

These results are again indicative of the diffe rent degree of stabilisation of the catchments of the two ponds. The trial pond catchmen t was largely stabilised, and had a relatively high pH and low suspended solids, whereas th e catchment of the upper pond included an area of bare clay, which resulted in lower pH and higher suspended solids.The Use of Flocculants and Coagulants to Aid Suspended Sediment Settlement in Earthworks Runoff - Trials, Methodology and Design

2509784 Beca Carter Hollings & Ferner Ltd Page 30 L1:90873-flocculation report - Becas Rev A Although the Floc Block was achieving some treatment during the low flows at the end of the stormwater inflow period, it is unlikely that there was significant treatment of the total storm flow.



### 5.3 

### Discussion The field study was constrained by the lack of an effective sediment forebay, and the high bedload of soil transported by stormwater, which clogged the flumes and interfered with the flow of stormwater passing the Floc Blocks. The bed load problems necessitated cleaning an d reinstallation of the flume system during and/or after each rainstorm and prevented any effective experimentation to develop methods of treating peak storm flows with Floc Blocks. The following sections summarise the info rmation obtained from the field trials.



### 5.4 Characteristics of Floc Block in Stormwater Treatment Conditions Floc Blocks are convenient for transportation, handling, and installation on difficult sites, however, physical cracking and breakdown of the Floc Blocks under shaded dry conditions is a matter of concern. It potentially results in the loss of pieces of unreacted Floc Block into the environment, and also forms in a non-standard shape of the Floc Block when it is later exposed to stormwater, which may increase the rate at which the Floc Blocks degrade if the surface area is enlarged. The Floc Block became very weak when buried in wet sediment. Water appeared to penetrate and weaken it considerably, making cleaning difficult. Observations made during the Floc Block trial indicated that cracking and breakdown of new Floc Block may be able to be prevented by soaking new Floc Blocks in water for a period to enable hydration of the outer layer to a degree that does not rapidly dry out and crack. Softening of Floc Blocks during periods of continued submergence in water can lead to problems because of the highly viscous nature of the softened block which adheres strongly to anything it comes into contact with, and the tendency of the softened material to flow through mesh. Soft Floc Block, flowing under the influence of gravity was a problem when suspended in the mouth of a large culvert where the Blocks were periodically submerged (Refer Figure 16). It is highly desirable to avoid exposure of Floc Blocks to bedload. Bedload materials adhere to the highly viscous Floc Blocks and inhibit the solubilisation of the Block by the stormwater. In circumstances with high bedload, the obstruction caused by the Floc Blocks may initiate deposition of bedload and clogging of stormwater flow paths.The Use of Flocculants and Coagulants to Aid Suspended Sediment Settlement in Earthworks Runoff - Trials, Methodology and Design

2509784 Beca Carter Hollings & Ferner Ltd Page 31 L1:90873-flocculation report - Becas Rev A Floc Blocks will perform best in the absence of bedload. On sites prone to bedload transport along stormwater channels, a forebay is adequate to retain bedload from intense rainstorms. It would be necessary to achieve optimal Floc Block treatment.



### 5 . 5 P o t e n t i a l P e r f o r m a n c e
It is important to achieve a solubilisation of Floc Block that produces a concentration within the optimum dose range. If under dosi ng occurs, there appears to be no effective treatment; i.e. 50% of the optimum dose does not result in removal of 50% of the suspended solids load. Although overdosing was resulted in poor trea tment performance in bench tests, there has been no indication of overdosing in any field trials.
Solubilisation of Floc Blocks into stormwater requiring treatment is optimised when there is a shallow turbulent flow around and over the Floc Blocks. In submerged conditions with a low current flow passing the Floc Blocks, insufficient polyacrylamide is released from the Bloc k to provide treatment of the stormwater. In prolonged submerged conditions such as in the cesspit trial, the Floc Block softened to a degree that resulted in flow of the Floc Block and adhesion to any surface it came into contact with. When placed in plastic mesh baskets flat across the bottom of a flume with a fall of about 1 in 10, the treatment potential for AN2 is about 2 l /s per new Floc Block for earthworks stormwater with a suspended solids concentration of more than 5,000mg/ L

. The use of battens on the bottom of the flume is a simple way to create turbulence in the Floc Block area, and to promote mixing of water that has been in contact with the Floc Blocks. The use of vertical Blocks in cages offers so me potential for treatment over a greater flow range than horizontally placed Blocks. But, the Greenhithe field trial was confounded by problems with continual bedload blocking of the small spaces around the Blocks, and with the softened Block material flowing vertica lly downwards to fill the bottom of the cage altering the flow dynamics around the Block. There may be potential for improving the treatment capacity of Floc Block by a combination of more sophisticated cages and careful control of the energy of stormwater flows during contact with the Blocks. However, the deformation of the Blocks as they soften and breakdown mitigates against sophisticated design.



### 5.6 Optimal Flow for Floc Block Treatment
A saturated earthworks site will produce stormwater flows of more than 60 L/s/ha under intense rainfall conditions. Stormwater flows from earthworks sites are extremely variable and it is difficult to provide an array of Floc Blocks that will provide optimal dosing forThe Use of Flocculants and Coagulants to Aid Suspended Sediment Settlement in Earthworks Runoff - Trials, Methodology and Design

2509784 Beca Carter Hollings & Ferner Ltd Page 32 L1:90873-flocculation report - Becas Rev A such variable flows. With large numbers of Blocks in a single channel system there could be some potential for overdosing in low flow conditions. The ideal operating condition for Floc Block treatment of earthworks stormwater appears to consist of exposure of the Block to water only during the storm flow period, with the Block being out of the water when there is no storm flow. It is desirable to design a Floc Block treatm ent system to treat a specified maximum flow, and to use a flow balancing system ahead of th e Floc Block treatment system to ensure that the maximum design flow is not exceeded. Th e flow balancing system could also perform the bedload removal function for catchments that produced bedload. A practical upper limit for Floc Blocks in a si ngle flume or flow path for the treatment of earthworks stormwater should be set at 10 Bloc ks because of the risk of overdosing low flows. This would require restriction of the maximum stormwater flow to 15 – 20 l /s per flume or flow path. A number of flumes could be used to treat stormwater from catchments producing higher maximum flows.The Use of Flocculants and Coagulants to Aid Suspended Sediment Settlement in Earthworks Runoff - Trials, Methodology and Design

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# 6 SITUATIONS WHERE CHEMICAL TREATMENT MAY NEED TO BE CONSIDERED

Note – need to include reference to floccula tion criteria – to be developed further and incorporated into TP The requirements for sediment retention pond s for earthworks areas are provided in TP90 ‘Erosion and Sediment Control’ (ARC 1999). The performance of ponds constructed according to the TP90 specifications is genera lly good, but a number of situations have been identified where chemical treatment ca n provide a marked improvement in sediment removal. These are:



#  Undersized Sediment Retention Pond Chemical treatment is an advantage when a pond of a required size cannot be constructed. This may occur because of topo graphical constraints, difficult foundation conditions, or the presence of natural habitat s of high ecological value.



#  Physical Characteristics of Sediment Retention Pond In some situations a pond of a required volume can be accommodated but the design of the pond cannot be optimised in terms of shape, depth, location of inlet and outlet, or energy attenuation of the inflow. Chem ical treatment can improve performance in such conditions.



#  Soils Type Some soil types produce suspended sediment in earthworks stormwater that has very poor settlement in a normal sediment re duction pond. For example, the ALPURT project had suspended solids concentrations between 1500 - 4200 mg/L in waters of sediment ponds after several days with no inflow to the ponds. Chemical treatment has been found to be effective with such soil types



### .


#  Sediment Generation Poten tial of Earthworks Area In earthworks areas with highly erodible soils or very steep or long steep slopes, there is a high risk of increased erosion and sediment runoff in rainstorms. Chemical treatment can be effective in removing the very high suspended solids loadings generated in such conditions.



#  Use of the Earthworks Site Some common uses of earthworks sites, particularly repeated machinery movements (for example on haul roads) can result in high sediment loadings in stormwater. Chemical treatment would improve sedime nt removal from stormwater in such conditions.The Use of Flocculants and Coagulants to Aid Suspended Sediment Settlement in Earthworks Runoff - Trials, Methodology and Design

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#  Performance of the Sediment Retention Pond If a sediment retention pond does not perform adequately, addition of chemical flocculants will improve performance.



#  Receiving Environment Sensitivity Chemical treatment provides a means of reducing the sediment discharge to highly sensitive receiving environments.



#  Cumulative Discharges The number of sediment retention pond discharges should be considered, as well as the assimilative capacity of the receiving environment.The Use of Flocculants and Coagulants to Aid Suspended Sediment Settlement in Earthworks Runoff - Trials, Methodology and Design

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# 7 SUMMARY

7.1 Advantages Of Aluminium Coagulant Treatment Analysis of long term rainfall and stormwat er suspended solids data obtained from the ALPURT trials shows that more than 60% of the sediment runoff from an earthworks site occurs during the two or three rainstorms per earthworks season which exceed 30mm in 24 hours. Rainstorms of sufficient size and intensity to overflow the retention pond, discharge disproportionately large sediment lo ad to the receiving environment. For a normal earthworks site, with a well designed sediment retention pond, and without any influence of the factors listed above, treatment with PAC would achieve the benefits summarised in Table 15.
Table 15. Advantages of PAC Treatment of Earthworks Runoff for Standard Catchments on an Earthworks Season Basis

Retention Pond Size Treatment 3% 2% Without PAC treatment Total sediment discharged to receiving water, tonnes (dry weight)/ha
Efficiency of sediment removal in pond (%) 5.8

81 9.2

69 With PAC treatment Total sediment discharged to receiving water, tonnes (dry weight)/ha
Efficiency of sediment removal in pond (%) 1.0

97 2.1

93
Chemical treatment results in a major improvem ent in the sediment removal efficiency for rainstorms that exceed the hydraulic capacity of the retention pond. This is shown in Table 15 by the increased quantity of sediment re moved by chemical treatment for the smaller pond sizes. If any of the factors listed in Section 6 occur to either increase sediment erosion, or decrease pond performance; chemical treatment will have an advantage over gravity settlement alone.



### 7.2 Advantages of the Use of Floc Blocks

To be completed