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 paper 
2509784 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 Council 

 
The Use of Flocculants and Coagulants to Aid The Settlement of Suspended Sediment in 

Earthworks Runoff – Trials, Methodology and Design 

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Table of Contents 

LIST OF FIGURES..............................................................................................................(iv) 

LIST OF TABLES ...............................................................................................................(iv) 

AKNOWLEDGEMENTS ......................................................................................................(v) 

ABSTRACT.........................................................................................................................(vi) 

1 INTRODUCTION ........................................................................................................... 1

1.1 ALPURT Trials...........................................................................................1

1.2 Greenhithe Trials .......................................................................................2

2 CHEMICAL FLOCCULANTS AND COAGULANTS ..................................................... 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 CHEMICAL DOSING SYSTEM................................................ 8

3.1 Introduction................................................................................................8

3.2 Design of Rainfall Driven 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 TRIALS...................................................................... 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.
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8 SUMMARY .................................................................................................................. 35

8.1 Advantages Of Aluminium Coagulant Treatment.............................35

8.2 Advantages of the Use of Floc Blocks...................................................35
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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 Required 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 and 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 Retention 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, 29/06/00 
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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. 
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1 INTRODUCTION 

Land contouring for urbanisation and infrastructure 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 ecosystems 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 related ecosystem impacts of many land-disturbing 

activities occurring within developing areas. Consequently, investigations have been 

undertaken to improve the existing control measures and construction methodologies, and 

develop new control measures. 

This report presents the results of trials undertaken 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. Flocculants 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. Due to the scale of the works and the potential 

effects of a number of sediment laden discharges 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 flocculation chemical as well as a rain activated 

system utilising fluid chemical were developed. 

Allied Colloids (Australia Pty Ltd.), through their New Zealand agent, Chemiplas NZ 

Limited, supplies three flocculants in solid form 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 chemical 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 sites. The Floc Blocks break down in flowing 

water, supplying chemical to the flow that causing 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. 
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In addition, a rain-activated system that triggers 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 proved 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 potential advantages in the use of Floc Blocks 

in comparison to the use of the rainfall activated 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 result, a more comprehensive study to determine 

the effectiveness of the Floc Blocks was initiated. 

The objective of this detailed field assessment 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 conditions to be expected from an earthworks 

site. 

A residential subdivision development was selected at Greenhithe. The Floc Block trial 

system was designed and installed before earthworks 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. 
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2 CHEMICAL FLOCCULANTS AND 

COAGULANTS 

2.1 Introduction 

Two types of chemicals were considered for initial bench testing and field trials as follows: 

 Polyelectrolyte flocculants; and 

 Aluminium coagulants, consisting of Aluminium 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 colloidal particles to remain in solution. 

Destabilising colloidal material to allow coagulation 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 settlement of coagulated material to occur as 

depicted in Figure 11. 

 

1 Reference: Fernz Chemicals (NZ) Ltd “A Guide to Water and Wastewater Treatment”, 

undated.

Negatively charged clay 

particles repel each other 

and remain in suspension

Colloidal particles are 

attracted to coagulant

molecules forming a floc 

that settles rapidly

Floc formed around 

coagulant molecule
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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 individual particles (Figure 2). This results in 

larger, denser flocs that settle more rapidly. 

Figure 2. Colloid Linking Mechanism2

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 charged flocculant commonly used for industrial 

applications including raw potable water clarification, and for clarification, thickening and 

dewatering of wastewater and sludge. Because these polymers have a high affinity for 

solids, the remaining concentration in treated waters 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 improve solids removal during pre-settlement. 

They are recognised as flocculants with greater toxicity implications for fish and other 

aquatic organisms than anionic or non-ionic polyelectrolytes. 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”, 

undated 
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Long polymer chains of polyacrylamide polyelectrolyte can degrade readily in aquatic 

systems. The oil fraction in liquid polyelectrolyte is readily degradable. Acrylamide, 

which is a raw material from which polyacrylamide polyelectrolytes are made, is highly 

toxic, and is reported by suppliers to be present 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 from 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 mixing 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 polyacrylamide 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 because 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 the 
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optimum dose range led to a reduction in flocculation 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 for potable water treatment and removal of 

sediment from stormwater. In the United States 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 suspended solids in sediment-laden runoff from 

ALPURT. 

2.5.2 Toxicity and Chemistry in Water 

The aluminium coagulants contain high concentrations of the toxic ionic form of 

aluminium. The USEPA freshwater ambient 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 phosphate precipitates. As they form these 

precipitates, they combine with phosphorus, suspended solids, metals, and other dissolved 

and suspended matter. The insoluble precipitates 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 solids and nutrients, the dissolved aluminium is 

still reduced very rapidly to a low concentration with no serious toxicity implications. 
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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 ponds used for coagulation testing during the 

ALPURT trial program, showed no toxicity during the coagulation and settlement periods. 

2.5.4 Chemistry of Deposited Sludge or Sediment 

Insoluble precipitates formed by the addition of Alum to water containing dissolved 

and/or suspended matter are stable. International research conducted on the toxicity and 

leaching characteristics of Alum sludge has 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 tightly 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. 
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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 installation 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 storage 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 chemical 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 dosing 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 catchment tray is used to displace the liquid 

treatment chemical from a storage or reservoir 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 falling onto saturated earthworks areas, and 60% 

of the rainfall onto the stabilised contributing catchment area will require treatment at the 

design dose rate. 
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Figure 3. Rainfall Activated Flocculation 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 

Reservoir 
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Figure 5. Rainfall Activated Flocculation 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 

tank

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 tray
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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. However, 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 earthworks catchment of 1 hectare and chemical 

treatment 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.
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3⁄4 Liquid PAC obtained from Fernz Chemicals (NZ) Ltd contains 10.1% of Al2O3 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 then assume runoff to be treated is the 

equivalent of 50mm of runoff 

3⁄4 For a 1 ha catchment, this is 500 m3 of stormwater runoff 

3⁄4 The volume of PAC required to treat 500 m3 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 m2

. 

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 

(m2)

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 Al2O3 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. 
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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 header 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/m2 of rainfall catchment tray. 

The ALPURT header tank design for small earthworks 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 the 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 will 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 approximately 86 mm of rain, giving a volume of 129 L 

from a constructed catchment of 1.5 m2

. 

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 analysis 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. 
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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 occurred, 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 event. 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. Water 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 storm, 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 monitored carefully during the first few rainfall 

events to check that the system is effective, and 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. 
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4 THE ALPURT TRIALS 

4.1 Introduction 

A trial pond was established at Bawden Rd (north of Albany) where significant cut and fill 

earthworks were taking place in the catchment. 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 carefully 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 limestone 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 most 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. 
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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 PAC 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 0.13 
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Samples were taken after 1 hour of settlement. The alkalinity was measured given the 

quantity of CaCO3 (g/m3

) in solution. Longer settlement would result in further reduction 

in suspended solids. The data in Table 3 shows 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 laden runoff was investigated extensively along 

the ALPURT project during the 1998/99 summer, and during the winter of 1999. A total of 

21 systems were used, with contributing catchments ranging between 0.5 and 15 hectares. 

The overall treatment efficiency of PAC treated 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 obvious reduction in pH in all ponds except 

Lonely Culvert. 
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Tables 5. Inflows and Discharges of PAC Treated Ponds 

Pond Time Inflow Outflow 

Flow 

L/s 

SS 

mg/L 

pH Al 

mg/L 

Flow 

L/s 

SS 

mg/

L 

pH Al 

mg/L 

Reduction 

of SS (%) 

1.3.8 Over 0850 12 238 8.97 0.084 9 53 6.66 0.026 77 

1135 20 253 9.97 0.077 12 55 6.79 0.068 78 

Lonely 0938 40 25830 6.83 0.052 3 266 7.62 0.072 98 

1045 15 13310 6.62 0.093 20 214 7.02 0.018 98 

21340 0918 8 399 8.78 0.25 3 40 6.56 0.016 89 

1110 7 2564 7.03 0.11 15 57 6.55 0.01 88 

D5 0910 6 2132 6.81 0.16 4 65 5.96 0.025 96 

1110 7 2564 7.03 0.11 4 56 5.47 0.01 97 

Untreated 1930 12 1571 7.88 0.22 4 1378 7.74 0.31 12 

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 concentrations 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 particles 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 removal 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. 
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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 from Deb's and Lonely appeared to be related 

to the very fine suspended solids in the discharges. 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 0.12 
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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 had 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. However 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 increase rapidly until the pH is reduced below 

5. At this stage it is desirable to avoid discharges having a pH below 5.5. 
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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, partially 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 the 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). 
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Figure 9: Floc Blocks within Channel between Forebay and Pond 

Figure 10: Pond Inlet Channel Full of Sediment 

As the earthworks area was gradually stabilised, 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 suspended solids removal. It is difficult to 
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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 flow through the treatment flume for any Floc 

Block system. A practical flow limit is approximately 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. throttling 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 less 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 potential for removal of suspended solids from 

stormwater with consistent quality, particularly 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 Floc Block form of polyacrylamide; and 

(2) Design of a robust dosing system for delivery of the chemical such that the system 

could be rapidly and easily transferred to other sites. 

The trial site selected consisted of a four-hectare area of pastoral farmland with an easy 

contour falling from south to north towards a tidal 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 divided 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. 
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The trial site was the sediment pond for the 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 difficulties 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 cage. This system provided for increasing 

exposure of the Floc Block surface to the stormwater 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 10m3

 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 5m3

. 

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 dry 
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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 periods could prevent this problem. It has also 

been noted that Blocks that have been in water 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 installation 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-channel of the flume were completely filled with 

soil, and the treatment flume was completely clogged (as shown in Figures 11 and 12). 

About 60m3 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 from 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 concentrations show very poor settlement. 

Although the Floc Block treatment system was overwhelmed by bedload, the trial pond 

had a lower SS than the other two ponds. 
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Very low pH of stormwater runoff from exposed 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 1l/s. Good flocculation was achieved with 2 vertical Floc Blocks in 

a flow of about 0.5l/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 the 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 stormwater 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 through the trial flume during February and 

March. 

April 2000.

At the beginning of April, 20% of the trial pond 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 site into the culvert discharging to the trial 

pond (Figure 13). 
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Heavy rain fell on 21 April 2000 (total of about 50mm) and stopped on 22 April 2000. 

Major erosion of the surface stormwater channels above the trial flume occurred, and a 

topsoil and clay bedload again entered the flume, 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 compared with the first stormwater runoff from the 

site, and a reduced suspended solids concentration. 

During the 21/22 April 2000 rainstorm, it was clear 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 April 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 baskets 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 quality 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 topsoiling 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 catchment 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. 
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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 culvert 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. The 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 water 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 the 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. 
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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 surface 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 close to the minimum decant level, and slight 

floc formation was slightly occurring at the 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 the mouth of the culvert was attributed to the 

low flow velocity of water passing the Floc Blocks. 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 different degree of stabilisation of the catchments 

of the two ponds. The trial pond catchment was largely stabilised, and had a relatively 

high pH and low suspended solids, whereas the catchment of the upper pond included an 

area of bare clay, which resulted in lower pH and higher suspended solids. 
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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 and 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 information 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. 
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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 dosing 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 treatment 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 Block 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 2l/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 some 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 vertically 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 for 
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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 treatment system to treat a specified maximum flow, 

and to use a flow balancing system ahead of the Floc Block treatment system to ensure that 

the maximum design flow is not exceeded. The flow balancing system could also perform 

the bedload removal function for catchments that produced bedload. 

A practical upper limit for Floc Blocks in a single flume or flow path for the treatment of 

earthworks stormwater should be set at 10 Blocks because of the risk of overdosing low 

flows. This would require restriction of the maximum stormwater flow to 15 – 20l/s per 

flume or flow path. A number of flumes could be used to treat stormwater from 

catchments producing higher maximum flows. 
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6 SITUATIONS WHERE CHEMICAL 

TREATMENT MAY NEED TO BE 

CONSIDERED 

Note – need to include reference to flocculation criteria – to be developed further and 

incorporated into TP 

The requirements for sediment retention ponds for earthworks areas are provided in TP90 

‘Erosion and Sediment Control’ (ARC 1999). The performance of ponds constructed 

according to the TP90 specifications is generally good, but a number of situations have 

been identified where chemical treatment can 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 topographical 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. Chemical 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 reduction 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 Potential 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 sediment removal from stormwater in such 

conditions. 
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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. 
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7 SUMMARY 

7.1 Advantages Of Aluminium Coagulant Treatment 

Analysis of long term rainfall and stormwater 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 load 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 

Treatment Retention Pond Size 

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 improvement 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 removed 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