TP  Overview of the Effects of Residual Flocculants on Aquatic Receiving Environments 

Executive Summary 

1. The ecotoxicological and environmental risk of releases of residual coagulants and 

flocculants is reviewed. Two broad groups of flocculants are considered. These are 

polyelectrolytes and inorganic aluminium salts. 

2. The overall conclusion is that there appears to be a small risk to the natural aquatic 

environment arising from potential losses of unbound residual flocculants from 

treatment ponds on construction sites. Impacts are likely to be low level and also 

likely to not be significant in relation to other factors which govern the health of 

aquatic communities. The benefit of reduced sediment levels in discharges is 

considered to outweigh the risk of any low level impacts attributable to residual 

flocculants. 

3. Management focus should remain on best management practices, including 

preliminary bench testing where necessary, to ensure that the most appropriate 

product is used in a particular application and that water treatment chemicals are 

applied in accordance with product specifications to achieve the target dosage range. 

4. It is considered that there is not a sufficient level of environmental risk to justify a 

programme of field testing of residual flocculants. In any event, such a programme 

is not achievable in respect of polyelectrolytes because these cannot usually be 

reliably measured in the field (ANZECC 2000). Furthermore there are no guideline 

trigger levels for polyelectrolytes (ANZECC 2000) that might enable field data to be 

interpreted in relation to acceptable or non-acceptable concentrations. 

5. Other key findings of the review can be summarised as follows: 

 Polyelectrolytes are reported to have low mammalian toxicity and are generally 

considered to be innocuous in field situations. Aluminium coagulants also 

generally pose a small environmental risk at normal field pH. 

 The potential toxicity of cationic polymers is well recognised but is seldom 

realised in the field. There is a large body of evidence indicating that these 

polymers are inactivated by sorption to naturally occurring dissolved and 

suspended organic matter and clays and their potential toxicity thereby greatly 

diminished, effectively to non significant levels. 

 Polyelectrolytes do not bioaccumulate, are highly biodegradable, and so are not 

persistent in the environment. Bound residues pose no hazard to sediment biota 

and do not break down into toxic residuals. 
 

 

TP  Overview of the Effects of Residual Flocculants on Aquatic Receiving Environments 

 Aluminium in flocculated sludges is tightly bound under a wide range of redox 

conditions and is stable other than in severely acidified situations. The latter 

conditions are unlikely to occur naturally or be induced by the use of aluminium 

flocculants which themselves reduce pH. 

 

 

TP  Overview of the Effects of Residual Flocculants on Aquatic Receiving Environments 

Table of Contents 

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

1.1 BACKGROUND ...........................................................................................................................1 

1.2 AUCKLAND REGIONAL COUNCIL OBJECTIVES ........................................................................................2 

2. METHODOLOGY .......................................................................................................................3

3 PRODUCTS IN USE ......................................................................................................................4

3.1 POLYELECTROLYTES ....................................................................................................................4 

3.2 ALUMINIUM COAGULANTS .............................................................................................................5 

4 POTENTIAL TOXICITY OF COAGULANTS AND FLOCCULANTS ...............................................................6

4.1 POLYELECTROLYTES ....................................................................................................................6 

4.2 ALUMINIUM COAGULANTS ........................................................................................................... 10 

4.3 PH...................................................................................................................................... 11 

5 RESIDUAL CONCENTRATIONS, ECOTOXICITY INDICATORS AND ACTION LEVELS .................................. 13

5.1 ALUM................................................................................................................................... 13 

5.2 PAC.................................................................................................................................... 13 

5.3 PAM................................................................................................................................... 14 

5.4 SUMMARY ............................................................................................................................. 14

6 OTHER CONSIDERATIONS...........................................................................................................16

7 CONCLUSIONS.......................................................................................................................... 17

REFERENCES................................................................................................................................... 20

Page 1TP  Overview of the Effects of Residual Flocculants on Aquatic Receiving Environments 

1 Introduction 

1.1 Background 

This report provides an overview of the ecotoxicological and environmental effects of 

residual coagulants and flocculants on natural waters. This overview is based on 

recent publications and scientific literature that has reviewed the environmental effects 

of the use of these chemicals. The report has been commissioned by the Auckland 

Regional Council (ARC) and has been prepared by Boffa Miskell Ltd. Beca Carter 

Hollings and Ferner Ltd have undertaken a review of the draft findings and completed 

the compilation of this report. 

The report is part of a two-stage study. Broadly, the first stage is a review of available 

information, the findings of which may lead to a second stage which would be more 

field orientated. Boffa Miskell Ltd was commissioned to undertake only the first stage 

at this point. 

Coagulants and flocculants can include polyelectrolytes and conventional inorganic 

coagulants such as aluminium and iron salts. They are used to remove particulate 

matter from a variety of waters. Polyelectrolytes are water-soluble polymers based on 

either synthetic or naturally occurring chemicals. Synthetic polyelectrolytes include 

polyacrylamides which can be in several charge states (cationic, anionic and non-ionic) 

and various cationic products, mainly polyamines. 

These chemicals have uses other than water clarification, such as dewatering of 

sludges and dispersants to stabilise suspensions (Murgatroyd et al. 1996). However, 

in the context of the current review, their use for water clarification is of primary 

interest. Specifically, it is the effects of their use as an aid in the removal of 

suspended sediment in earthworks runoff that is of interest. Their discharge in site 

effluents and clarified waters, to drains, creeks, streams and marine zones are issues 

of concern to the ARC. 

While some of the specific uses of flocculants (e.g. as process chemicals in industrial 

applications) are of little direct relevance to the present review, many of these 

chemicals have multiple applications and generic qualities. The studies of potential 

toxicity of these chemicals allow an informed view to be developed of the risk to the 

receiving environment as it applies within the Auckland region. 

The Polyelectrolyte Suppliers Group, which is a subcommittee of the chemical sector 

of the Plastics and Chemical Industries Association (PACIA), have produced a 

Responsible Care Guideline for use of Polyelectrolytes Guidelines (PACIA 1998). 

Those Guidelines provide a code of practice for the use of polyelectrolytes and the 
 

 

Page 2 TP  Overview of the Effects of Residual Flocculants on Aquatic Receiving Environments 

assessment of environmental risks. It comments that polyelectrolytes are reported in 

the literature to have a very low mammalian toxicity and are generally considered to be 

innocuous materials (PACIA 1998). 

However, in some circumstances (such as overdosing with chemical flocculants and 

otherwise inappropriate use which can potentially lead to the release of unbound 

flocculants), there is a potential and perceived environmental risk. It is this aspect that 

this review addresses. 

1.2 Auckland Regional Council Objectives 

The ARC has identified the following objectives (paraphrased) in the overall project 

description as a guide to issues that needed to be addressed: 

 Identify whether flocculated ponds are discharging significant amounts of residual 

flocculant. 

 Identify effects on stream life arising from such discharges. 

 Investigate the effects of cumulative discharges of flocculants. 

 Evaluate other factors that may affect the performance of flocculant systems (e.g. 

soil pH, rainfall and lime stabilisation). 

 Identify aquatic species sensitive to flocculants. 

 Comment on the effects of flocculant discharges to the marine environment. 

 If necessary, undertake field trials to investigate residual flocculants and their 

effects. 
 

 

Page 3 TP  Overview of the Effects of Residual Flocculants on Aquatic Receiving Environments 

2. Methodology 

A search for information available on the Internet was undertaken. Water and 

wastewater treatment companies and local authorities were contacted regarding 

flocculant products used in their processes, however, their response was generally 

poor. Chemical supply companies were contacted directly and information obtained on 

flocculant products, including Material Safety Data Sheets (MSDS). A search of the 

library catalogues of the Auckland Regional Council library, Auckland City Libraries, and 

University of Auckland library system was undertaken. The Biological Sciences library 

was also searched for relevant information. 

The specific tasks undertaken for this part of the study were: 

 Compile a list of coagulant and flocculant products or product types in 

use. Compile a dossier of product information including Material Safety 

Data Sheets (MSDS), where available. 

 Review ecotoxicological information and if applicable compare dosed and 

residual concentrations with ecotoxicity indicators or action levels. 

 Provide conclusions on: 

– Environmental risk (low/moderate/high) 

– Identified at-risk animals/plants 

– Bioaccumulation and environmental persistence 

– Recommendations for further investigations. 
 

 

Page 4 TP  Overview of the Effects of Residual Flocculants on Aquatic Receiving Environments 

3 Products in Use 

3.1 Polyelectrolytes 

Polyelectrolytes are water-soluble macromolecules used in polymer treatment 

systems on construction sites. They include both primary coagulants and flocculant 

aids. These are briefly discussed below although for the purposes of this review the 

distinction is not important. 

3.1.1 Coagulants 

These are usually cationic (positively charged) and are used in lieu of aluminium and 

iron salt coagulants. They act by destabilising suspensions by overcoming the 

repulsion between particles by neutralising the surface charges, allowing particles to 

adhere and subsequently be removed by settlement. Primary coagulants are mainly 

polyamines. 

3.1.2 Flocculants 

These are mainly polyacrylamides (PAMS). PAMS is a generic term. PAMS are made 

up of many subunits of the monomer acrylamide. Chemical and physical properties 

differ greatly depending on the specific characteristics of the polymer. 

The current generation of PAMS are reported to be ultrahigh molecular weight and 

moderately anionic (negatively charged) although they can come in various charge 

states. They are used to cause stabilised particles or coagulated suspensions to form 

aggregates that can be removed by settlement or filtration. 

Different polyelectrolytes have different uses, benefits and limitations. For example, in 

a draft technical publication prepared for the ARC which presents the findings of field 

trials on chemical removal of sediment from earthworks stormwater, Larcombe (1999) 

notes that cationic (positively charged) PAMS are commonly used in municipal 

wastewater treatment to improve solids removal during pre-settlement. They are also 

cited as being used in clarification of drinking water and industrial applications including 

waste and sludge clarification, thickening, dewatering, paper manufacture and mining 

(Goodrich et al 1991). 

It is also noted that the use of groups of these products having different charge 

classes is not mutually exclusive. For example, an anionic flocculant can be used after 

a cationic coagulant and serves to neutralise any unreacted coagulant. The actual type 

and dosage of polymer needed to flocculate sediments is dependant on the types and 
 

 

Page 5 TP  Overview of the Effects of Residual Flocculants on Aquatic Receiving Environments 

levels of sediment in the water. For example, cationic polymers are reported generally 

to create a more complete flocculation when removing colloidal clays from water 

(Gannon, undated). 

Polyelectrolyte products, or representative product types available are listed in 

Attachment A. 

3.2 Aluminium Coagulants 

Aluminium coagulants include aluminium sulphate (alum) and polyaluminium chloride 

(PAC). Larcombe (1999) comments that inorganic aluminium coagulants have been 

extensively used in the United States for direct dosing of eutrophic lakes which has 

achieved significant reductions in suspended matter, nutrients, oxygen demand and 

turbidity. 

Choice of coagulant is dependant on a range of factors that must be assessed on a 

case specific basis. For example Larcombe (1999) found that alum treatment achieved 

good sediment removal during storm conditions when the hydraulic capacity of a trial 

retention pond was exceeded. Thus it could be considered a fairly robust product in 

that application. PAC is a less acidic product and may be preferred in naturally acidic 

locations. Larcombe found in field trials that PAC had consistently less effect in terms 

of pH reduction and concluded that it would have a lesser pH influence in sensitive 

receiving waters. Floc blocs suffered a number of limitations, but were determined to 

be effective in small catchments when flow balancing could be achieved before 

treatment and stormwater runoff was of a consistent quality. 

Thus, choice of coagulant is determined partly by the capability of the chemical but 

also by extrinsic factors such as catchment size, receiving environment sensitivity, 

stormwater quality, etc. 

Larcombe (1999) describes the general action of these coagulants as follows. Addition 

of the aluminium coagulant to water results in dissolved aluminium ions being 

incorporated into aluminium hydroxide and aluminium phosphate precipitates. These 

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

and suspended matter. The insoluble precipitates are stable and as particle size 

increases so does density and the aggregates sink. 

Aluminium products or representative product types are listed in Attachment B. 
 

 

Page 6 TP  Overview of the Effects of Residual Flocculants on Aquatic Receiving Environments 

4 Potential Toxicity of Coagulants and 

Flocculants 

4.1 Polyelectrolytes 

4.1.1 Toxic Action 

Despite 30 years of use there are few public and peer reviewed data on the toxicity of 

polymeric flocculants (ANZECC 2000). Toxicity varies with charge type and flocculant 

chemistry. The cationic polymers are recognized to pose a potentially significant 

hazard to aquatic life, particularly fish, but this varies with crustaceans (ANZECC 2000). 

The surface of fish gills carries a negative charge to which cationic polyelectrolytes will 

readily bind due to electrostatic attraction (Murgatroyd et al. 1996). It is likely that 

sublethal effects and mortality of fish is the result of mechanical suffocation, reduction 

in oxygen transfer and mucous production on gill surfaces. Impacts on invertebrates 

are likely to derive from a similar mechanism (that is, interactions with the surfaces of 

aquatic organisms). 

Freshwater Impacts 

The toxicity of polyelectrolytes to freshwater organisms varies widely, with reported 

EC50/LC50 values between 0.04->4,000 mg/l, depending upon ionic state and the 

particular organisms concerned (e.g.. algae, invertebrates or fish). Cationic polymers 

are more toxic to aquatic life than anionic or non-ionic polyelectrolytes. For example 

the median EC50/LC50 values cited by Murgatroyd et al (1996) for invertebrates and 

fish were 3.9 and 0.89 mg/l respectively. The varying toxicity from cationic 

polyelectrolytes is explained by differences in charge density, with toxicity increasing 

as charge density increases. 

The non-ionic polymers are considered to be the least toxic. Non-ionic polymers and 

anionic (i.e. negatively charged) polymers are considered to be the safest and easiest 

to use (often being in solid form) in environmentally sensitive situations because of 

their low toxicity. 

Marine Impacts 

Murgatroyd et al. (1996) in a review, provide a summary of polyelectrolyte toxicity to 

marine organisms based on very few studies. The data is cited as laboratory based 

acute studies in which no ‘natural substances’ such as humic or fluvic acids and clays 

were added. Therefore the results were considered to represent worse case 

estimates with respect to aquatic toxicity. That is, much higher field concentrations 
 

 

Page 7 TP  Overview of the Effects of Residual Flocculants on Aquatic Receiving Environments 

could be required to generate a toxic effect but it is likely such concentrations would 

occur in natural waters. 

Notwithstanding these limitations, the EC50/LC50 range for cationic products is cited 

as 1000-2370 mg/l for fish and for anionic products for fish is cited as >1000 mg/l. No 

data is presented for non-ionic products. Although the marine studies are so few as to 

negate valid comparison with the freshwater data, it is noted that the marine values 

are 1-2 orders of magnitude greater than comparable values for freshwater fish 

presented by the same authors, which showed median EC50/LC50 of 0.89 and 37.2 

mg/l for cationic and anionic products respectively. 

The literature accessed in the current review provides no other data or peer reviewed 

published studies on the effect of residual flocculants in the marine environment. 

Apart from the study cited above, the toxicological and bioassay studies cited have 

focused on freshwater species or non-marine species of alga and bacteria. These 

studies confirm a low level of biological risk in field situations. 

Notwithstanding this, there is a strong basis upon which to extrapolate the findings of 

the non-marine studies in general to the marine environment. Of particular relevance 

are the generic characteristics of flocculants and their propensity for toxicity to be 

lessened by particulate and dissolved organic matter including humic substances, and 

by neutral range pH. Saline water carries a strong signature of these characteristics 

and it is therefore likely that residual flocculants would be rapidly inactivated. Such 

bound residuals are stable and do not release or breakdown into toxic components. 

Additional to this is the reality that any discharges of residual flocculants are likely to be 

highly infrequent and of a very small volume relative to the dilution potential of the 

receiving waters (especially in the coastal marine area). 

All in all, notwithstanding the lack of empirical information, there is no reasonable basis 

upon which to speculate that residual unbound flocculant is likely to cause adverse 

effects in the marine receiving environment. 

4.1.2 Bioaccumulation 

Polyelectrolytes are generally of high molecular weights that do not pass across 

biological membranes. Therefore bioaccumulation does not occur. 

4.1.3 Persistence 

Polyelectrolytes are highly biodegradable. Biodegradation and abiotic hydrolysis are 

important processes contributing to the removal and deactivation of polyelectrolytes, 

especially in alkaline conditions. In a natural water body complete degradation of 

cationic polymers can be expected to occur within a few weeks. There is little data 

available on degradation of non-ionic and anionic polymers but the potential toxicity of 

these is low and is not of concern. 
 

 

Page 8 TP  Overview of the Effects of Residual Flocculants on Aquatic Receiving Environments 

Sorption of the potentially more hazardous cationic polyelectrolytes to dissolved and 

suspended organic matter and clays is recognized as a particularly important 

mechanism for the removal of cationic polymers from the water column and for a 

greatly diminished toxicity risk. Furthermore such binding is thought to be irreversible 

and bound residues pose no hazard to sediment dwelling biota. 

In summary, cationic polyelectrolytes are considered to have a low persistence. 

4.1.4 Residual Acrylamide 

Polyacrylamides contain residual acrylamide that may be released into the surface 

waters. Acrylamide monomer (AMD) is a neurotoxin. Murgatroyd et al. (1996) noted 

that polyacrylamide appeared to be significantly more toxic to invertebrates in the field 

than would be indicated by laboratory studies of acute exposure. The cause of the 

toxicity in this case was considered to probably be via a sub-lethal mechanism such as 

a neurotoxicological effect. Murgatroyd et al. (1996) concluded that the environmental 

risk of acrylamide is unclear because of lack of data. 

However Larcombe (1999) comments that although acrylamide is toxic in field 

situations, it is present in low concentrations and is completely degraded by bacteria in 

surface waters. Sojka and Surapaneni (2001) comment that it is considered to be 

environmentally safe at levels present in polyacrylamide products. Guidelines for PAM 

use in the United States recommend products with <0.05% AMD. AMD is easily 

metabolised by micro-organisms in soil and biologically active waters and has a half-life 

of tens of hours (Lande et al., 1979; Shanker et al., 1990) 

4.1.5 Other Relevant Comments on Toxicity 

Polyacrylamide (PAM) is described by Sojka and Lentz (1996) as “an environmentally 

safe industrial flocculant widely used in municipal water treatment, paper 

manufacturing, food processing and other sensitive applications”. 

PAM is used extensively in agriculture in the United States to reduce soil erosion 

(Sojka and Sudapaneni, 2001). Research in Idaho described by Yonts and Benham 

(1998) found that less than 5 percent of PAM applied in furrow irrigation left the field in 

runoff water, and this quickly fell below detection limits (at distances of >1500 yards). 

These authors state that there was no indication of any adverse impact on soil, plant or 

aquatic systems when anionic PAM was used for erosion control in this application. 

Tobiason et al. (2001) note that effective doses for water and soil treatment are 

generally an order of magnitude or more below toxic concentrations. Bioassays using 

daphnids exposed to test-plot runoff treated with 80 mg/l wet-applied PAM solution 

found no acute or chronic effects attributable to PAM. 

In a report on potential use of PAM in Australian agriculture, Sojka and Surapaneni 

(2001) state that environmental and safety considerations of anionic PAM's have been 
 

 

Page 9 TP  Overview of the Effects of Residual Flocculants on Aquatic Receiving Environments 

thoroughly reviewed by Barvenik (1994), Bologna et al. (1999), and Seybold (1994), and 

conclude that no significant negative impacts have been reported for aquatic macrofauna, edaphic micro-organisms or crop species when used for erosion control at 

recommended concentrations and rates. Even in high dose applications effects on 

biota are greatly buffered due to adsorption and deactivation associated with 

suspended particles. 

Cary et al. (1987) in bioassay experiments on the freshwater daphnid (Daphnia magna) 

and the freshwater fish fathead minnow (Pimophales promelas) reported bentonite and 

all of the dissolved organic carbon compounds tested (humic, fulvic, and tannic acids, 

lignin and lignosite) reduced the toxicities of cationic polymers by one to two orders of 

magnitude. In that study levels of suspended solids and dissolved organic carbon 

were representative of environmental conditions. The authors concluded that the 

results of acute toxicity tests conducted in standard reconstituted laboratory water 

may be too stringent for estimating the effects of cationic polyelectrolytes in receiving 

waters. In other words, they suggested that laboratory studies generally overestimate 

toxicity effects likely to be experienced in the field. 

Goodrich et al. (1991) in laboratory bioassay experiments of rainbow trout 

(Onchorhynchus mykiss) similarly found that a strong negative correlation existed 

between toxicity of cationic polyelectrolytes and humic acid, whereby as humic acid 

concentration increased toxicity greatly decreased. Residual concentrations tend to 

quickly adhere to particles in the receiving waters or to channel surfaces. These 

authors concluded that the physical nature of the receiving waters would determine 

the actual acute response, in that case for trout. They emphasised that toxicity should 

be evaluated not only in standardised tests but also under conditions of actual use to 

obtain reasonable estimates of hazard. Any such studies of toxicity in field situations 

have not been reported. 

Despite good suspended solids removal rates in bench tests, Larcombe (1999) 

reported major difficulties with the practical use of PAM in field studies of the Albany 

to Puhoi State Highway 1 motorway extension project (ALPURT) earthworks 

stormwater. Larcombe commented that because of the high affinity for solids of both 

anionic and cationic polymers, the remaining concentration in treated waters is very 

low in all but serious overdose situations. Such low concentrations would effectively 

avoid a toxicological risk. 

Murgatroyd et al. (1996) make a pertinent comment with respect to analysis, to the 

effect that analytical methods have limits of detection typically in the order of 1mg/l 

but methods are not specific and are prone to interference from other substances. 

The implication is that measurement of field concentrations of residual polyelectrolyte 

is difficult and this situation makes equally difficult controlled toxicity evaluations in the 

field. 
 

 

Page 10 TP  Overview of the Effects of Residual Flocculants on Aquatic Receiving Environments 

4.2 Aluminium Coagulants 

4.2.1 Toxic Action 

Aluminium coagulants contain high concentrations of ionic aluminium, the toxic form. 

Toxicity is very dependant on pH and increases at lower pH. At high pH, most 

aluminium is present in solid form and is not bio-available. Below pH 6 it is mostly in 

the dissolved, bio-available form. The bioavailability and toxicity of aluminium is 

generally greatest in acid solutions and generally most toxic over the pH range 4.4 -5.4 

with a maximum toxicity around 5.0-5.2 (ANZECC 2000). However, at pH of 6.5–8.0, 

which is the normal range for natural waters, there is generally considered to be little 

threat of toxicity. Where pH of natural waters is outside this range, instream values 

including fish and invertebrate diversity would tend to be limited in any event, and 

environmental sensitivity reduced. 

In fish the toxic effect is manifest as an accumulation of mucous on gill surfaces, 

which impairs their function. Aluminium has been implicated in fish mortalities in 

acidified waters (Baker, 1982). A review by Spry and Weiner (1991) concluded that in 

low-pH water (i.e. 6-6.5 or less): “both sub-lethal and lethal toxicity of aluminium has 

been clearly demonstrated in both laboratory and field studies at environmental 

concentrations”. 

Complexing agents including humic substances reduce the bioavailability of aluminium 

to organisms resulting in lower toxicity. 

Larcombe (1999) reported that even at doses in excess of requirements, the dissolved 

toxic aluminium is reduced in the receiving environment very rapidly to a very low 

concentration with no serious toxicity implications. Larcombe concluded from dosing 

trials that the risk of significant adverse effects from discharges associated with 

aluminium based flocculant materials is low. 

While no studies have reported the effect of salinity on the uptake and toxicity of 

aluminium to estuarine and marine animals, aluminium is relatively insoluble in 

seawater, where pH normally ranges from 6.5 to 8. Under these conditions insoluble 

Al (OH)3

 is the predominant species and the dissolved aluminium species 

concentrations, which are responsible for both toxicity and bioavailablity, are likely to 

be low. Discharges containing high concentrations of potentially toxic dissolved 

aluminium, as might be associated with acidified runoff, should be rapidly rendered 

non-toxic on entering the marine zone provided dilutions are significant and mixing is 

relatively rapid.

4.2.2 Bioaccumulation 

Conflicting results have been reported on the effect of pH and uptake of aluminium, 

and hence its bioavailability, in freshwater organisms (ANZECC 2000). Aluminium 
 

 

Page 11 TP  Overview of the Effects of Residual Flocculants on Aquatic Receiving Environments 

forms stable complexes with natural dissolved organic matter (DOM) and this greatly 

ameliorates its bioavailability. Under a normal pH range of 6.5 to 9 for freshwaters and 

6.5 to 8 for marine waters, aluminium can be considered to carry a low risk of 

bioaccumulation. Runoff from acid soils will tend to enhance bioaccumulation risk if 

the effect is to reduce pH to below 6.5. Conversely, runoff from alkaline soils will tend 

to increase pH and thereby reduce such risk. However, in addition to mediating 

influences such as DOM, other factors need to be taken into account on a site specific 

basis. For example in an 'acid catchment' the biota may already be limited by or at 

least robust, in relation to exposure to dissolved metals. It seems unlikely that small 

scale episodic releases of aluminium based residual flocculant diluted with general 

catchment runoff would significantly alter the bioaccumulation risk.

4.2.3 Persistence 

The biogeochemical cycle of aluminium is complex, yet poorly understood (ANZECC 

2000). Research into effects of alum sludge discharged from wastewater treatment 

plants indicates that dissolved aluminium can be released from sludge under highly 

acidic conditions (George et. al., 1991). However overseas research indicates that 

alum sludge is stable under normal pH conditions and pollutants have little or no 

affinity for release. 

Furthermore, aluminium is reported to be tightly bound in alum treated sediment under 

both reduced and oxidized conditions and at pH ranges between 5-7 (Larcombe, 1999). 

Larcombe (1999) reports that alum floc is not toxic to benthic organisms and small 

planktonic crustaceans present in sediment pond water showed no toxicity during 

coagulation and settlement periods. 

Aluminium toxicity is reduced by calcium and dissolved organic carbon. The use of 

lime for soil stabilisation would have the effect of increasing water pH and thereby 

decrease aluminium toxicity. There is also evidence of acclimation (i.e. increase in 

resistance or tolerance) of fish to aluminium (Spry and Weiner, 1991). Fish in waters 

with high background concentrations of aluminium may therefore have enhanced 

resistance to episodic increases. 

4.3 pH 

Both polyacrylamides and aluminium flocculants can lower water pH, which can have 

direct pH effects and also potentially increase the toxicity of metals through making 

them more bio-available. The mechanism for this bio-availability is of some interest 

because it emphasises that the alkalinity of the receiving waters has an important 

bearing on toxicity potential. 

In a study of metal bio-availability and toxicity to fish in low alkalinity lakes, Spry and 

Weiner (1991) commented that the greater bioaccumulation potential comes not only 
 

 

Page 12 TP  Overview of the Effects of Residual Flocculants on Aquatic Receiving Environments 

from the greater aqueous abundance of metals in such waters but also as a result of 

low concentrations of aqueous calcium increasing the permeability of biological 

membranes to metals. This emphasises that the quality and character of the receiving 

water (e.g. its hardness) can have an important influence on toxicity potential. 

The responses to pH of nine fish species and one shrimp native to New Zealand 

freshwaters were investigated by West et al. (1997). Most of the species preferred 

pH values between 6.5 to 9.5, although shortfinned elvers (Anguilla australis), koaro 

(Galaxias brevipinnis), and banded kokopu (G. fasciatus) did not avoid water of low pH. 

The response of most fish to pH within the normal range for streams appears to be 

weak. 

Where potentially toxic concentrations of heavy metals are present, low pH could 

potentially facilitate adverse effects on biota by increasing the dissolved, bioavailable 

fraction of metals and therefore increasing the toxicity of metals. Where metal 

concentrations are below guideline concentrations in the receiving environment, toxic 

effects would be unlikely irrespective of the pH. In other words, for pH to be a 

potentially important factor, the environment into which the residual flocculant is to be 

discharged would have to be heavily polluted in the first instance. 

Effects of pH are also discussed by Harding et al. (2000). They note that New Zealand 

invertebrates appear to be relatively tolerant, but that molluscs are likely to be a 

relatively sensitive group. Changes in pH can also reduce or increase periphyton 

biomass, but such effects are poorly understood. Notwithstanding this, because few 

steams in the Auckland Region are stony–bottomed and primarily periphyton driven, 

such effects are unlikely to be significant. 

The concentration of dissolved aluminium increases as pH lowers below 6.5. 

Larcombe (1999) observed that concentrations did not increase rapidly until pH 

decreased below 5. Larcombe (1999) recommended that discharges from treatment 

devices such as settlement ponds, having a pH below 5.5 should be avoided. 

However, Larcombe (1999) concluded that even in relatively acidic clay soils, flocculant 

dosing is unlikely to reduce pH to such levels. There is a risk associated with 

overdosing, and appropriate management or quality assurance procedures should be 

instituted to minimise such risks. 
 

 

Page 13 TP  Overview of the Effects of Residual Flocculants on Aquatic Receiving Environments 

5 Residual Concentrations, Ecotoxicity 

Indicators and Action Levels 

ANZECC (2000) reports that there are insufficient data to develop guideline trigger 

levels for polyelectrolytes, particularly given the range of polymer types. ANZECC 

(2000) recommended that because polyelectrolytes cannot usually be measured in 

water (particularly in field situations), discharges are best controlled by best 

management practices and other appropriate source controls. 

At a local level, a series of trials of several aluminium based flocculant products was 

undertaken by Larcombe (1999), to determine practicality of application, optimal doses, 

effectiveness of sediment removal, effects on pH, and concentrations of dissolved 

aluminium in bench tests and discharges from stormwater treatment ponds. Relevant 

results are summarised below. 

5.1 Alum 

Alum was tested in a catchment with major cut and fill in limestone (Onerahi chaos) 

soils. The aluminium dose rate was 5.5 mg/l. Dissolved aluminium in the discharge 

from the retention pond averaged about 0.1mg/l, and was often lower than the 

dissolved aluminium concentration at the inflow to the pond from the site. The runoff 

pH at the site was between 7.9 and 8.2, and the treatment reduced the pH by about 

0.5 pH units (Larcombe, 1999). 

Bench testing undertaken by Larcombe (1999) 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 1 hour settlement period. 

5.2 PAC 

Because of the presence of clay soils with high natural acidity in the ALPURT highway 

project the use of polyaluminium chloride was investigated (PAC is less acidic than 

alum). 

The dissolved aluminium concentrations in the outflows from treated ponds ranged 

from 0.010 – 0.084 mg/l, and were lower in most of the pond outflows than in the 

inflows. Concentrations were lower than the applicable USEPA criteria. The 

dissolved aluminium concentration in an untreated control pond was much higher. 
 

 

Page 14 TP  Overview of the Effects of Residual Flocculants on Aquatic Receiving Environments 

Bench tests confirmed that PAC had consistently less effect in reducing pH than alum. 

Trials for stormwater ponds showed an obvious reduction in three of the four treated 

ponds. Inflow concentrations ranged from 6.81 to 9.97, and outflow concentrations 

from 5.47 to 7.62 mg/l. 

5.3 PAM 

Bench testing of a number of liquid polyacrylamide (PAM) formulations achieved good 

removal of suspended solids from ALPURT sector B1 stormwater but major difficulties 

were identified with the use of liquid PAM for dosing of stormwater on remote sites. 

These problems arose primarily due to the highly viscous form of liquid PAM 

concentrates. This required pre-dilution of the concentrates, which generated the 

need for special mixing equipment, storage tanks and electric power. Pre-diluted PAM 

still retained a high viscosity and had a limited storage life. 

A solid form (the Floc Bloc) was tested but problems were also encountered. These 

problems included blocs deteriorating, flow channels in which the blocs were deployed 

becoming clogged, and difficulty in achieving effective dosing under storm flows. 

Sokja and Lentz (1996) carried out studies, which also emphasised the inherent 

difficulties but importance of achieving uniform PAM dissolution and distribution. They 

focused on the use of PAM as a conservation measure mitigating irrigation induced 

erosion, reporting on PAM use where small amounts of PAM were dissolved in 

irrigation water used in furrow irrigation in the U.S.A. In that case the PAM copolymer 

utilised was a dry granulate which was used to prepare a liquid stock solution. The 

authors noted that similar products are available worldwide. The PAM’s were metered 

into the irrigation supply ditches either as a concentrated stock solution, or as dry 

granules. 

The authors emphasised that if dry granules were used then methods needed to also 

be used to promote the uniform dissolution and distribution of PAM. They 

emphasised that no untreated water should wet the furrows ahead of the flows 

treated with PAM, as this greatly reduced the PAM’s effect. While the findings of 

Sokja and Lentz (1996) confirm the effectiveness of PAM’s in that particular application 

they also emphasise the general situation that effectiveness is very much a function of 

the ability to control dosage and delivery systems. 

5.4 Summary 

In conclusion, the bench tests and field trials reported by Larcombe (1999) showed 

that alum had a greater effect on pH than PAC. Outflow concentrations of aluminium 

for the PAC trials were generally lower than the inflows and lower than USEPA chronic 

toxicity criteria (0.087 mg/l, 4 day average not to be exceeded in waters with pH 6.5-

9.0). 
 

 

Page 15 TP  Overview of the Effects of Residual Flocculants on Aquatic Receiving Environments 

Sensitivity of native biota to pH and heavy metals is discussed by Harding et al. (2000) 

and Hickey (2000). New Zealand biota generally appear to be tolerant to these 

stressors although there is some evidence of effects in streams receiving mine 

drainage. Molluscs are identified as potentially sensitive to low pH. The most 

sensitive organisms to metals were the cladoceran Ceriodaphnia dubia and amphipod 

Paracalliope fluviatilis, while the mayfly Deleatidium sp. was fairly sensitive. Sensitivity 

of New Zealand aquatic insect species has not been assessed in specific relation to 

aluminium. 

However, from the limited available studies on other metal species, it appears unlikely 

that any species or groups of key ecological significance will be sensitive to potential 

toxicants released from treated ponds. 

Lime stabilisation works would tend to increase pH and therefore buffer the effects of 

flocculants on pH, and also reduce aluminium toxicity. 
 

 

Page 16 TP  Overview of the Effects of Residual Flocculants on Aquatic Receiving Environments 

6 Other Considerations 

In addition to the complexities of the variable chemistry associated with the wide 

range of flocculants, the factors governing the potential for effects are themselves 

complex. Such effects include: soil type; the spatio/temporal proximity of discharges; 

natural discharge and dilution scenarios; the specific ecological characteristics of local 

receiving environments; the natural mitigative chemistry of the receiving environment 

that would deactivate flocculants (e.g. total organic carbon, suspended and colloidal 

material and clay surfaces), and the value placed on those waters by the community. 

Where there are concerns regarding multiple and perhaps cumulative discharges, 

further investigations or monitoring might at first glance seem warranted. However, 

given the problematic analysis of polyelectrolytes and the general absence of any 

strong body of evidence establishing significant adverse effects in the field, best 

management practice and source controls would still seem the most cost effective 

way of ensuring that chemical treatment avoids adverse off-site water quality effects. 
 

 

Page 17 TP  Overview of the Effects of Residual Flocculants on Aquatic Receiving Environments 

7 Conclusions 

In light of their low toxicity, anionic and non-ionic polymers are recognised as the 

safest to use and would be the most appropriate where a particular receiving 

environment was regarded as a sensitive location. Sensitivity could be based on a 

variety of values including ecological (e.g. important native fish stocks); chemical (e.g. 

high natural acidity); cultural; or perceptual. 

Cationic polymers are widely recognised as having a high toxicity under laboratory 

conditions in pure waters. However there is a large body of evidence verifying that 

under natural conditions this toxicity is greatly mitigated by natural organic compounds 

and natural surfaces. 

In relation to the ARC Objectives outlined in Section 1.2 of this report, the following 

summary comments can be made at this point. 

3⁄4 Identify whether flocculated ponds are discharging significant amounts of 

residual flocculant. 

This has not been empirically determined for polyelectrolytes at this point. Based on 

the review of the literature including the Larcombe (1999) work, it is unlikely that 

significant quantities of unattached (residual) flocculant are released from treatment 

ponds, provided that organic polymer treatment chemicals or aluminium based 

flocculants are being used as prescribed for the particular product in use. 

In so far as polyelectrolytes are concerned, there are in any event insufficiently well 

developed analytical techniques to enable levels of discharged flocculant to be reliably 

identified let alone measured. 

In laboratory studies, acute effects have been shown to be the main response. 

However, in relation to dissolved aluminium, Larcombe (1999) found that 

concentrations in the outflow of treated ponds were below the USEPA chronic 

criterion and therefore also well below the acute threshold. 

3⁄4 Identify effects on stream life arising from such discharges. 

It follows from the above that this cannot be done directly for polyelectrolytes. The 

literature provides no strong body of evidence documenting adverse effects in the field 

or data that would suggest such effects are likely. Significant effects are unlikely to be 

occurring and therefore are not a significant issue. 

3⁄4 Investigate the effects of cumulative discharges of flocculants. 

The same comments apply. 
 

 

Page 18 TP  Overview of the Effects of Residual Flocculants on Aquatic Receiving Environments 

3⁄4 Evaluate other factors that may affect the performance of flocculant systems 

(e.g. soil pH, rainfall and lime stabilisation). 

Many factors can affect the performance of flocculants. These need to be assessed 

on a case by case basis in relation to the particular capabilities and applications of 

individual products, of which there are many. Most importantly the type and dose of 

polyelectrolyte required for a particular application needs to be determined by either 

laboratory tests (e.g. jar tests) or site-specific field trials or both. These would take into 

account the ability to deliver the flocculant to the target water body under the 

expected range of hydraulic and water quality conditions. Lime stabilisation works 

would tend to increase pH and therefore buffer the effects of flocculants on pH, and 

also reduce aluminium toxicity in relation to the particular characteristics of the 

particulates to be settled. No general conclusions can be offered that would be of 

assistance in the context of the present review. 

3⁄4 Identify aquatic species sensitive to flocculants. 

There has been no specific systematic comparative study of New Zealand species. 

Overseas laboratory based research suggests that fish are the most potentially 

sensitive group and cationic polymers are the flocculant group of most concern. 

Crustaceans have also been shown to be sensitive but like fish only under laboratory 

conditions. 

3⁄4 Comment on the effects of flocculant discharges to the marine environment. 

In the ARC region, stormwater treatment ponds may frequently be close to estuarine 

and marine receiving environments. 

Murgatroyd et al. (1996) in a review, provide a summary of polyelectrolyte toxicity to 

marine organisms based on very few studies. The data is cited as laboratory based 

acute studies in which no ‘natural substances’ such as humic or fulvic acids and clays 

were added. Therefore the results were considered to represent worse case 

estimates with respect to aquatic toxicity. That is, much higher field concentrations 

could be required to generate a toxic effect but it is likely such concentrations would 

occur in natural waters. 

Notwithstanding these limitations, the EC50/LC50 range for cationic products is cited 

as 1000-2370 mg/l for fish and for anionic products for fish is cited as >1000mg/l. No 

data is presented for non-ionic products. Although the marine studies are so few as to 

negate valid comparison with the freshwater data, it is noted that the marine values 

are 1-2 orders of magnitude greater than comparable values for freshwater fish 

presented by the same authors, which showed median EC50/LC50 of 0.89 and 37.2 

mg/l for cationic and anionic products respectively. 
 

 

Page 19 TP  Overview of the Effects of Residual Flocculants on Aquatic Receiving Environments 

The literature accessed in the current review provides no other data or peer reviewed 

published studies on the effect of residual flocculants in the marine environment. 

Apart from the study cited above the toxicological and bioassay studies that have been 

cited have focused on freshwater species or non-marine species of alga and bacteria. 

These studies confirm a low level of biological risk in field situations. 

Notwithstanding this, there is a strong basis upon which to extrapolate the findings of 

the non-marine studies in general to the marine environment. Of particular relevance 

are the generic characteristics of flocculants and their propensity for toxicity to be 

lessened by particulate and dissolved organic matter including humic substances, and 

by neutral range pH. Saline water carries a strong signature of these characteristics 

and it is therefore likely that residual flocculants would be rapidly inactivated. Such 

bound residuals are stable and do not release or breakdown into toxic components. 

Additional to this is the reality that any discharges of residual flocculants are likely to be 

highly infrequent and of a very small volume relative to the dilution potential of the 

receiving waters (especially in the coastal marine area). 

All in all, notwithstanding the lack of empirical information, there is no reasonable basis 

upon which to speculate that residual unbound flocculant is likely to cause adverse 

effects in the marine receiving environment. 

3⁄4 If necessary, undertake field trials to investigate residual flocculants and their 

effects. 

Field trials to determine concentrations of unbound residual flocculant and/or to assess 

toxicity response are not recommended and are considered unwarranted based on the 

low level of environmental risk associated with residual flocculant releases. 
 

 

Page 20 TP  Overview of the Effects of Residual Flocculants on Aquatic Receiving Environments 

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 Appendix A 

Polyelectrolyte Product Types 

Technical Publication  Section x  Page 4

 Appendix B 

Representative Aluminium Product Types