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

3 Impact assessment of substitution of phosphates on wastewater treatment plants

By substituting phosphorus in laundry detergents with phosphate-free builders, the phosphorus inlet concentration is expected to be significantly reduced and concentrations of replacement builders will increase. According to SPT 60-70% of the laundry detergents and 100% of automatic dishwashing detergents today contains phosphate, which means that a full substitution of phosphate with alternative complex building chemicals will result in significant increased load of these substances[6].

In this chapter, the expected load changes of phosphorus and alternative replacement substances will be determined and the effects on the wastewater treatment plant operation and performance will be discussed.

3.1 Compositions, assumptions and approximations

3.1.1 Composition of phosphate-containing and phosphate-free detergent builder systems

According to SPT phosphate-containing laundry detergents (powder) constitutes 60-70% of the total market volume in Denmark while all detergent products for automatic dishwashing detergents (washing tabs and powder) contain phosphates today. While the builder system in laundry powder detergents and automatic dishwashing detergents that contain phosphate is quite similar, the composition of phosphate-free detergents in laundry powder and automatic dishwashing detergents is more different. See Appendix A for prototypes of phosphate-free and phosphate-containing laundry detergents and products for automatic dishwashing.

Most importantly, phosphate-free laundry powder detergents are mainly built on inorganic zeolite while the automatic dishwashing phosphate-free detergents are built on organic acids/salts and inorganic silicates and carbon. The content of the inorganic mineral zeolite in phosphate-free washing powder is so significant that it will have an impact on the wastewater composition at the inlet of wastewater treatment plants (see Section 3.4).

Phosphorus discharge from phosphate-containing laundry detergents can be estimating assuming a certain unit consumption of detergents (laundry powder) from each of the 2,532,000 households in Denmark (Statistics Denmark, 2007). Based on Statistics Denmark (2007) 1,094,000 persons are living alone in Denmark and if it is assumed that they perform 150 machine washes per year using 70 gram of washing powder per wash they will consume 11,487 tonnes of washing powder per year. Accordingly 1,438,000 families was registered in Denmark in 2006 and assuming that they perform 400 machine washes per year families in Denmark would consume 40,264 tonnes of washing powder per year. This results in a total consumption of 51,751 tonnes washing powder per year. This amount is higher than estimated in (Danish EPA, 2001) (28,700 tonnes/year) but lower than given in (Statistics Denmark, 2006) (78,364 tonnes/year).

The amount of phosphorus in laundry powder detergents is calculated based on the assumption that 90% of the detergents discharged by private households in Denmark reach the wastewater treatment plants, that 65% of the detergents contain phosphorus and that the weight percentage of phosphorus in washing powder is 5% (based on 25% P in phosphate). This results in a total phosphorus discharge from laundry powder detergents on 1,514 tonnes P/year.

Phosphorus discharge from phosphate-containing automatic dishwashing detergents has been estimating assuming a certain unit consumption of detergents (automatic dishwashing powder) from each of the 1,438,000 families in Denmark (Statistics Denmark, 2007). It is estimated that the 1,438,000 families perform 300 machine washes per year using 22 gram of automatic dishwashing powder per wash. It is assumed that the 1,094,000 single living persons do not use automatic dishwashing machines in their home. This results in a total consumption of 9,491 tonnes automatic dishwashing powder per year. This amount is also higher than estimated in (Danish EPA, 2001) (3,800 tonnes/year) but lower than given in (Statistics Denmark, 2006) (15,005 tonnes/year). The amount of phosphorus in automatic dishwashing powder detergents is calculated based on the assumption that 90% of the detergents reach the wastewater treatment plants, that 100% of the detergents contain phosphorus and that the weight percentage of phosphorus in washing powder is 11% (based on 25% P in phosphate). This results in a total phosphorus discharge from automatic dishwashing detergents on 960 tonnes P/year.

3.1.2 Phosphorus loads to wastewater treatment plants

According to Danish EPA (2004) Denmark has 1,193 municipal wastewater treatment plants with a capacity higher than 30 PE (2004). Most part of the wastewater (90%) is treated at 263 larger wastewater treatment plants that receive approximately 600 mill. m³/year in total. The average inlet concentration of phosphorus to the larger wastewater treatment plants can based on this be calculated to 8.9 mg P/L, which results in a total phosphorus load to the wastewater treatment plant on 5,350 tonnes P/year (Danish EPA, 2004).

The phosphorus load to Danish wastewater treatment plants has been reduced significantly over the last 10-20 years. At Lynetten wastewater treatment plant, the phosphorus load has been reduced from 1,100 tonnes P in 1988 to 750 tonnes P in 1995. In 2006, Lynetten wastewater treatment plant received 509 tonnes/year (Lynetten, 2006), which resulted in an average phosphorus inlet concentration on 7.8 mg P/L. A major reason for the reduction of phosphorus load is partly believed to be caused by a reduced use of phosphorus-containing laundry detergents in private households and reduced industrial activity in the catchment area of Lynetten.

In Table 3-1, the major phosphorus load sources to the 263 largest wastewater treatment plants are listed. The major phosphorus sources come from human excretion (urine and faeces), detergents (laundry and automatic dishwashing) and other sources (other household load institutional and industrial sources).

Assuming that the population connected to the wastewater treatment plants excretes 0.5 kg P/year (Jönsson et. al, 2005) and that 80% of the total phosphorus generation of the human discharge reaches the wastewater treatment plant this will result in a phosphorus load to the wastewater treatment plants on 1,940 tonnes P/year or 36% of the total phosphorus load to the wastewater treatment plants.

The amount of phosphorus originating from household detergents (laundry and automatic dishwashing was estimated to 2,475 tonnes P/year which constitutes 46% of the total phosphorus load today.

The remaining amount of phosphorus 935 tonnes P/year (18%) is assumed to originate from industrial sources (phosphorus discharged after internal pre-treatment) and from institutions other than human excretion.

Table 3-1 Phosphorus load to the 263 largest wastewater treatment plants in Denmark

Pollution source Discharge Unit
Human excretion 1,940¹) Tonnes P/year
Phosphorus-containing laundry detergents 1,514²) Tonnes P/year
Phosphorus-containing automatic dishwashing detergents 961³) Tonnes P/year
Other sources 935 Tonnes P/year
Total phosphorus load to wastewater treatment plants 5,3504) Tonnes P/year

  1. Assuming 80% discharge of total load from human phosphorus excretion will reach the wastewater treatment plants
  2. Assuming 51,751 tonnes laundry detergents is discharged to the sewer system (65% phosphate-containing detergents) with a phosphorus (P) weight percent on 5 g/kg detergent
  3. Assuming 9,491 tonnes automatic dishwashing detergents is discharged to the sewer system (100% phosphate-containing detergents) with a phosphorus (P) weight percent on 11 g/kg detergent
  4. Phosphorus inlet load to the 263 largest wastewater treatment plants in Denmark, 2004 (Danish EPA, 2004)

3.2 Expected change in loads to wastewater treatment plants of substances originating from laundry and automatic dishwashing detergents

3.2.1 Change in phosphorus load to wastewater treatment plants

Assuming a total ban of phosphorus containing detergents for laundry and automatic dishwashing will be adopted, this will have a great impact on the phosphorus load to wastewater plant in Denmark. Assuming the load estimations of phosphorus to Danish wastewater treatment plant will be reduced with 2,475 tonnes P/year, the total load reduction of phosphorus will change from 5,350 tonnes P/year to 2,875 tonnes P/year. This will reduce the average inlet concentration of phosphorus from 8.9 mg P/L to 4.8 mg P/L (calculations based on 2004 data, adapted from Danish EPA (2004)).

3.2.2 Change in phosphate-free detergent load to wastewater treatment plants

Zeolites are volumetrically the most predominant chemical in phosphate-free detergent builder products. Furthermore, zeolites (zeolite A) are insoluble inorganic hydrated aluminosilicates (Na12(AlO2)12(SiO2)12×27H2O that interacts and becomes a part of the biological sludge at wastewater treatment plants. Consequently, zeolite is the most relevant ingredient to focus on when determining load and impact of phosphate-free detergents on wastewater treatment plant performance and operation (Piirtola et al., 1998).

No records have been found on the presence of zeolite mineral in inlet wastewater to Danish wastewater treatment plants but if it is assumed that 35% of all washing powder detergents presently contain zeolite a total discharge on 4,075 tonnes zeolite to the wastewater treatment plants must presently be expected, which equals to an average inlet concentration on 6.8 mg zeolite/L. Assuming that phosphate-containing detergents will be replaced 100% with zeolite-containing builder products, a total load on 11,600 tonnes of zeolite will be discharged to municipal wastewater plants, which will increase the inlet concentration of zeolite to 19.3 mg/L. These numbers are in good correlation with Hopping (1978), Maki (1978) and Morse et al. (1994) who reported zeolite inlet concentrations in the range 10-30 mg/L. However, the inlet concentration of zeolite is very much depending on the hardness of groundwater, which is reflected by Fischer et al. (1978) who determined zeolite concentrations up to 60 mg/L in hard water areas of Germany.

3.3 Effects on the wastewater treatment plant operation as a result of reduced phosphorus load

3.3.1 Phosphorus removal in biological and chemical wastewater treatment processes

Today phosphorus is being removed efficiently at most Danish wastewater treatment plant by a combination of biological phosphorus removal and chemical precipitation of phosphorus. The biological processes require phosphorus for bacterial growth of heterotrophic as well as autotrophic bacteria. Since heterotrophic processes are the most predominant in terms of concentration and growth rate of biomass, the phosphorus requirement is normally calculated for this group of bacteria. According to Henze et. al (2000) the phosphorus requirement constitutes 1.5% of the sludge production, which normally account for 1/3 of the total phosphorus removal in municipal wastewater treatment plants (Gert Petersen, 2007). E.g. for removal of 500 mg COD/L the phosphorus requirement for the biological heterotrophic growth process would be 3.4 mg P/L for a normally observed yield of the process on 0.45 kg CODB/kg CODS.

Phosphorus is also being removed in the so-called Bio-P process where a special group of bacteria (mainly Acinetobacter) are able to accumulate a higher amount of phosphorus as storage products internally. This process is becoming more and more attractive in Denmark as a “green” alternative to traditional precipitation with chemicals since the additional phosphorus accumulation is removed with the excess sludge production without using chemicals. It is estimated that 30-40% of the total phosphorus removal in Denmark today takes place as a result of the Bio-P process (Gert Petersen, 2007).

Chemical precipitation with ferrous or aluminium metal salts is the most traditional and widely used method of removing phosphorus in wastewater treatment processes. The advantage of using these metal salts for phosphorus removal is that the phosphorus removal efficiency can be controlled quite accurate and the effluent concentration of soluble phosphorus can be reduced to almost zero. The efficiency of the chemical phosphorus removal process is determined by the molar ratio between phosphorus and the metal salt. Normally a molar ratio of 1.0 mole Me/mole P is necessary to reduce the effluent phosphorus to 1.5 mg TP/L but in order to reduce tax payment of phosphorus discharge most Danish wastewater treatment plants try to obtain a phosphorus concentration below 0.5 mg P/L in the effluent, which requires a molar ratio of 1.5 mole Me/mole P. The result of the increased molar ratio is an increase in the sludge production on 50% (chemical sludge).

3.3.2 Phosphorus shortage for the biological processes

It is evident that a reduction of the phosphorus load will have a great impact on how wastewater treatment plants will be operated in the future. In case that a total ban of using phosphorus detergents for washing and automatic dishwashing in private households will be determined, the reduction of the phosphorus load to wastewater treatment plants will almost be reduced to half (46%), which means that most of the phosphorus can be removed biological and chemical sludge production will be minimized significantly. In fact, phosphorus shortage could be causing problems to some wastewater treatment plants that presently have low phosphorus concentrations in the inlet compared to the COD inlet concentration and as a result of that phosphorus has to be added to the biological processes on these wastewater treatment plants.

3.3.3 Sludge composition and production

A major benefit of a reduced precipitation with chemicals is the reduction in the chemical sludge production at the wastewater treatment plants. As a rule of thump removal of 1 kg phosphorus results in production of 15 kg suspended solids with a molar ratio between metal and phosphorus on 1.5 (Gert Petersen, 2007). Assuming 30% of all phosphorus is being removed chemically the potential reduction of chemical sludge production will be approximately 24,000 tonnes of sludge per year, which is about ¼ of the total sludge production in Denmark. However, the net reduction in sludge production will be much less because the substituted chemical in detergents (primarily zeolites) will contribute to the sludge production (Section 3.4.1).

3.3.4 Chemical savings

Another significant effect of a diminished chemical precipitation is the reduced use of chemical addition in the wastewater treatment plant. It is estimated that 40,000-50,000 tonnes precipitation chemical solution (iron chloride, iron sulphate and aluminium chloride primarily) can be saved as a result of the significantly reduced precipitation with chemicals, which will reduce the operation costs of Danish wastewater treatment plants in the order of 50 mill. DKK per year. Even though the need for phosphorus precipitation in theory can be prevented, chemical precipitation cannot be avoided in practice when the effluent concentration of soluble phosphorus should be treated down to very low concentrations (below 0.5 mg Psoluble/L). The chemical consumption will however be much lower because of the much lower phosphorus amounts that needs to be removed.

3.3.5 Increased volatile fatty acids (VFA) for denitrification

Biological phosphorus removal (Bio-P) will be reduced in case the full reduction of the phosphorus load will take place (46% reduction in the phosphorus load). This will reduce the need and efficiency of the Bio-P process since the phosphorus concentration level in the wastewater will be much lower. As the Bio-P process consumes large amount of VFA, the VFA consumption will be reduced, which can benefit the denitrification process. Hence, the nitrogen removal that consumes easily degradable organic matter will potentially be improved.

3.4 Expected effects on the wastewater treatment plant operation as a result of using phosphate-free detergents in private households

Substitution of phosphate builders with alternative builder products will result in a change in the sludge composition and behaviour on the wastewater treatment plants. 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%) and as mentioned earlier zeolite is expected to have the most predominant impact on the wastewater treatment plant performance. In the following focus is on the expected effects caused by a higher zeolite concentration in the incoming wastewater to the wastewater treatment plants.

3.4.1 Increased content of zeolite in the sludge composition

Zeolite will as an inert particulate organic compound accumulate in the biological sludge and displace a part of the organic fraction of the sludge. The actual build-up of zeolites depends on the specific plant operation – i.e. the mixed liqueur suspended sludge concentration (MLSS), the load of zeolites and the sludge production. Hence, it is difficult to state precisely how big a part of the organic sludge fraction the inorganic zeolites will drive out. Piirtola et al. (1998) reported an increase in sludge suspended solids of up to 25-30% in pilot tests where zeolite was added in concentrations that reflect the expected concentration increase of zeolite when substituted with phosphate builders in detergents. The result of this is that the MLSS concentration in the aeration tanks must be increased in order to avoid loss of nitrification at the plants. For wastewater treatment plants operated with simultaneously phosphorus precipitation, i.e. addition of precipitation chemicals directly in the MLSS, there might be status quo or even a reduction in the inorganic fraction of the sludge, which means that these wastewater treatment plants could be run with a lower MLSS set point in the aeration tank.

3.4.2 Effects on biological processes

Several of studies have been investigating the effect from zeolite on the nitrification process (Piirtola et al., 1998, Wahlberg, 1995, Fischer et al, 1978, Hopping, 1978). The main conclusions are that zeolite did not inhibit nitrification in any cases when mixed with sewage. In fact, Fischer et al. (1978) reported an improvement of the nitrification process achieved by longer sludge retention and lower sludge loading. Another indirect effect that in some cases can improve nitrification is the production in alkalinity that occurs if zeolite undergoes hydrolysis to form kaolinite (Al4Si4O10(OH)8).

An interesting observation by Wahlberg (1995) stated that phosphate-containing detergents produced smaller amounts of surfactants, lower toxicity, lower concentration of suspended solids and lower oxygen concentration at the wastewater treatment plants. In fact, that oxygen consumption expressed as BOD7 was in average three times higher and the suspended solids concentration ten times higher for zeolite containing washing waters than for phosphate-containing washing waters, which would have a great impact for the aeration processes at the wastewater treatment plants.

3.4.3 Reduced hydraulic capacity of secondary clarification tanks

For wastewater treatment plants that will experience an increase of the MLSS concentration in the aeration tanks, the sludge loading of the secondary clarification tanks will increase. Depending on the existing hydraulic capacity, this could result in a decrease of the hydraulic capacity of the plant, since the clarifiers capacity is determined by the amount of sludge that passes them. This phenomenon will probably only affect the wastewater treatment plants that do not perform significant simultaneous precipitation (i.e. they do not have any chemical sludge in the biological sludge) and as a consequence will experience a reduced VSS/SS (Volatile Suspended Solids/Suspended Solids) ratio .

3.4.4 Sludge settling characteristics

Zeolite is a particulate non-degradable substance with a higher density compared with activated sludge flocs. Hence, zeolite will have a positive weighting action on the biological sludge, which will increase the sedimentation velocity of sludge in the settling tank (Piirtola et al., 1998). Better settling characteristics of sludge will result in improved hydraulic capacity of clarification tanks and less floating sludge discharge in the outlet.

3.4.5 Reduced filtration of sludge

The improved weighting action can be hindered by floc breakdown caused by the ion-exchange capacity of zeolite. This can result in creation of an amorphous form of zeolite in the sludge matrix, which can be harmful for sludge filtration (Cook et al., 1982). In Piirtola et al. (1998) this phenomenon was investigated in a pilot scale activated sludge process and it was shown that long contact times (on more than 2 hours) deteriorated filterability of the sludge with more than 40% - probably caused by the complexing metal ions of zeolite – or more likely – by its amorphousness structure of sludge that zeolite creates. Reduced filterability of sludge can result in significant higher sludge volumes after the sludge dewatering process, probably depending on the sludge dewatering technology (centrifuges, belt presses etc.). However, it must be expected that the use of existing or new polymer chemicals partly can account for this problem and reduce the negative effect of reduced filterability of the sludge.

3.5 Conclusions

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. Presently 60-70% of laundry detergents and 100% of automatic dishwashing detergents still contains phosphate and 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 that constitutes 5,350 tonnes phosphorus per year.

Presently, the average phosphorus concentration level in wastewater is 8.9 mg P/l (2004) and this level is expected to be reduced to 4.8 mg P/L 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. Furthermore, a significant saving of precipitation chemicals results in a reduction the operation costs in the order of 50 mill. DKK per year. The lower phosphorus concentration is not expected to affect the biological processes – only in cases where the absolute removal of COD is high (more than 800 mg COD/L removed biologically) and the phosphorus concentration is low (4-5 mg P/L) phosphorus shortage could be observed and as a result of this deterioration of the biological performance at the plant.

The sludge composition at wastewater treatment plants will with simultaneous phosphorus precipitation – at first – change by an increase in the VSS/SS ratio, which favours the biological and hydraulic performance of the plants since the presence of chemical sludge virtually can be avoided. However, one of the replacement substances in phosphate-free detergents – zeolite – is expected to affect the sludge composition and sludge behaviour significantly, because zeolite (an inorganic undegradable particulate compound) 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.

The characteristics of sludge settling and dewatering will be affected by the significant increased content of zeolite in the sludge. While the settling characteristics will probably be improved at some plants due to the increased density of zeolite, sludge dewatering will deteriorate vastly unless sludge dewatering chemicals suitable for zeolite containing sludge will be developed/used. 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 being developed.

In general, modifications of processes as well as alterations of buildings and other constructions at the wastewater treatment plant may be necessary and consequently expenses may be expected in this regards.

There is no indication that phosphate-free detergents (zeolites) in any way will inhibit the biological processes – in particular the nitrification process.

[6] 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, hence the consequences from a ban of phosphate will be less significant than assumed based on the 2007 numbers.

 

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Version 1.0 December 2009, © Danish Environmental Protection Agency