Summary and conclusions

Assessment of the consequences of a possible ban on phosphates in household detergents

A proposal on limitation on usage of phosphates in household detergents may be released by the EU Commission or proposed as part of the HELCOM Baltic Sea Action Plan. The purpose of this project is to carry out an environmental impact assessment of the potential substitutes for phosphates in laundry detergents and to assess the consequences on the wastewater treatment from the substitution of phosphate and the increased use of the alternative substances.

The project identifies substances for substitution of phosphate in laundry and automatic dishwashing detergents. The most important substitutes for phosphates in laundry detergents are zeolites, polycarboxylates, citrates and phosphonates. New alternatives as IminoDiSuccinate (IDS), Methyl Glycin Diacetic Acid (MGDA) and GLutamic acid Diacetic Acid (GLDA) are also introduced in builder systems today but to a smaller extend. Most important as alternatives to phosphates in automatic dishwashing products are citrates, silicates and carbonates.

An environmental assessment was carried out for the aquatic environment as a stepwise process. The environmental assessment did not include effects on the terrestrial environment. First an initial screening identified the substances with the highest hazard potential. The screening was based on typical concentration of the phosphate alternatives in laundry detergents and automatic dishwashing products. Parameters from the Detergent Ingredients Database list (DID list) from the European and Nordic ecolabels were used to calculate a risk index which indicates the environmental hazard potential of the substances. For the substances with the highest hazard potential, a determination of predicted environmental concentrations, PEC was carried out by use of two concepts, EUSES and ECO Lab modeling. Ecotoxicological data was found in available databases and the Predicted No Effect Concentrations (PNEC_aquatic) were derived according to the principles in the EU Technical Guidance Document for risk assessment (TGD). The risk quotients, RQ as the PEC/PNEC ratio were determined as the result of the environmental risk assessment for chronic as well as for acute exposure.

The screening identifies sodium silicate and zeolite as the substitutes for phosphate in laundry detergents with the highest environmental hazard potential. For automatic dishwashing products sodium silicate is the phosphate alternative with the highest hazard potential based on results of the screening.

An aquatic risk assessment was conducted for sodium silicate, zeolite, phosphonates and IDS (Iminodisuccinate) with focus on their use as substitutes for phosphates in laundry detergents. The risk quotient (RQ) calculated for chronic exposure and for acute exposure were all calculated to be below 1 indicating that use of the phosphate alternatives in laundry detergents is not expected to pose a risk to the aquatic environment.

The consequences on the wastewater treatment plant operation and performance of an expected change in the load of phosphorus and replacement with alternative substances were evaluated.

The calculation of the phosphorous load on Danish wastewater treatment plants was determined on the basis of estimations of household consumption of detergents in Denmark. When comparing this calculation of household consumption of detergents with that of the Danish EPA, a significantly larger amount of detergents was found (51,751 tonnes detergents/year as compared to 28,700 tonnes/year as assumed by the Danish EPA in 2001). When these figures are compared with those from Statistics Denmark, both estimates are much lower since the Statistics Denmark has estimated a household consumption of 78,364 tonnes/year. These differing figures indicate that the estimations of the actual consumption of detergents in Danish households are subject to significant uncertainty.

By introducing a phosphate ban of laundry and automatic dishwashing detergents (powder and tabs) in private households, the phosphate concentration level in municipal wastewater will be significantly reduced since. Presently 60-70% of washing detergents and 100% of automatic dishwashing detergents still contains phosphate[²]. Assuming that the Danish households consume 51.751 tonnes detergents/year it is estimated that the reduction in phosphate load to wastewater treatment plants could be as high as 46% - a reduction of 2,450 tonnes of phosphorus per year. The phosphorus concentration level in wastewater of 8.9 mg P/L is expected to be reduced to 4.8 mg P/L in average as a consequence of a full phosphate substitution in household detergents.

The impacts of this on the wastewater treatment plant operation is comprehensive since it probably will result in an almost complete diminishment of chemical precipitation processes, which today accounts for 30-40% of the total phosphorus removal and is responsible for a significant amount of the sludge production at Danish wastewater treatment plants. A significant saving of precipitation chemicals results in a reduction of the operation costs in the order of 50 mill. DKK per year. However, modifications of processes and alterations of buildings and other constructions at the wastewater treatment plant may be needed. The lower phosphorus concentration is in general not expected to affect the biological processes.

The volumetric load of alternative builder products to wastewater treatment plants will mainly constitute zeolite (25%), sodium carbonate (20%), sodium silicate (8%) and sodium citrate (7%). Zeolite is expected to have the most predominant impact on the wastewater treatment plant performance. Zeolite will accumulate in the sludge matrix with up to 25-30%, depending on the loading to the wastewater treatment plant. This will reduce the organic fraction of the sludge and counteract on the reduced chemical precipitation in the biological process. Investigations have shown that sludge dewatering can be reduced by at least 40%, which will increase the sludge volumes with the same amount unless methods to dewater sludge as efficiently as today are developed. However, this phenomenon is rather complicated and not very well documented in the literature. The reduced sludge dewatering of zeolite-containing sludge has been documented by several investigations, but none of them have investigated how this phenomenon can be counteracted, e.g. by addition of sludge polymers in the sludge treatment operation. For the past 10-20 years, the development of sludge-dewatering polymers has shown that a change in the chemistry of sludge has a remarkable effect on the reduction of sludge volumes from wastewater treatment plants and it must be assumed that this development will also benefit zeolite-containing sludge. Reduced sludge-dewatering conditions will naturally lead to increased sludge volumes but since the addition of precipitation chemicals will be significantly reduced, this problem might never arise.

Within the framework of this project, it has not been possible to clarify the actual conditions at wastewater treatment plants receiving zeolite-containing wastewater. It is probably very difficult to determine the actual effect on the wastewater treatment plants since this effect most probably is determined by many factors like wastewater composition, the zeolite load, the alkalinity (hardness) of the water as well as the process conditions on the wastewater treatment plants. It might be possible to compare wastewater treatment plants that are comparable with respect to the biological process they perform, their load and the sludge treatment they conduct and one wastewater treatment plant receiving zeolite-containing wastewater (zeolite-containing detergents) and the other receiving wastewater without zeolite (phosphate-containing detergents). However, such a comparison requires a larger number of comparable wastewater treatment plants in order to be statistically relevant to evaluate.

In order to investigate the phenomenon thoroughly, it is recommended to conduct actual experiments under well-defined conditions (laboratory and pilot scale investigations under controlled conditions) including both wastewater and sludge treatment investigations.

There are no references indicating that phosphate-free detergents (zeolites) in any way inhibit the biological processes of the wastewater treatment plants.

Conclusions

Based on the results of the screening and the risk assessment of the substances with highest risk a ban of phosphate in laundry detergents will not lead to toxic effects in the aquatic environment.

Effects on Danish wastewater treatment plants from a ban of phosphate will most likely result the in lower operation costs in terms of less use of precipitation chemicals and a lower sludge production assuming that the resulting effect of sludge dewatering will not be seriously affected by the presence of zeolite. However expenses may be needed for modifications of processes and alterations of buildings and other constructions at the wastewater treatment plant.

[2] Information on the use of phosphate in detergents for laundry and automatic dish wash were collected during 2007. According to SPT the use of phosphate in both types of detergents are lower in 2009.