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Phosphate Recovery

PERSPECTIVES FOR P RECOVERY OFFERED BY ENHANCED BIOLOGICAL P REMOVAL

Jonathan Strickland

Innovation Technologist, Anglian Water Services, Henderson House, Lancaster Way, Ermine Business Park, Huntingdon, Cambridgeshire, PE18 6XQ

ABSTRACT

The current position with respect to phosphorus removal in a UK water company is discussed. The factors that would favour a change from the current practice of metal salt dosing to Biological removal are examined. The potential for therefore recovering phosphorus fixed biologically is considered and possible process routes presented.

KEYWORDS

Phosphorus, Nutrients, Activated Sludge, Biological Phosphorus Removal, Struvite

INTRODUCTION

Anglian Water is geographically the largest of the 10 Water Companies of England and Wales and also has the largest requirement for P removal under the Urban Waste Water Treatment Directive. By the end of 1998 sewage works of one million population equivalent in Eastern England will be subject to phosphorus discharge standards, amounting to around 20% of the connected population.

The method used to achieve this phosphorus removal initially will be metal dosing usually with Ferrous (Fe II) salts. In 1999 the water industry England and Wales will enter its third five year regulatory period, and this is expected to include a further requirement for Phosphorus removal probably doubling the population served in Eastern England.

There is therefore now sufficient Phosphorus available for recovery at sewage works for this to be viable, but a number of obstacles exist before this potential can be realised.

CURRENT PRACTICES

A decision was made in the mid 1990s that the current requirement for phosphorus removal in Anglian Water would be met using metal salt dosing. This was based on a number of considerations.

  1. Around half the works requiring phosphorus removal utilise trickling filters for either a part or all of the flow. Only metal salt dosing can be used to remove phosphorus in these cases.
  2. Of the works using activated sludge as a part or the only treatment process, a significant number co settled waste activated sludge in the primary tanks. If biological phosphorus removal was applied in those cases, any Phosphorus accumulated biologically would be released in the primary tanks and recycled.
  3. The works which had a significant portion of activated sludge and were suitable for conversion to biological phosphorus removal also utilised anaerobic sludge digestion. This was believed to cause the release of phosphorus into the liquor and following sludge thickening, recycling of phosphorus once again to the works inlet.

BIOLOGICAL PHOSPHORUS REMOVAL

Research on Biological Phosphorus removal in Anglian Water has indicated the following :-

  1. Phosphorus can be successfully removed from wastewater using mainstream Bio P configurations.
  2. In a minority of cases this removal can be maintained year round without any additional chemical dosing.
  3. For most sites backup chemical dosing of some sort would be required during the winter months to maintain removal.
  4. The process can be operated with the normal level of process control currently employed in Anglian Water.
  5. Storage of waste activated sludge resulted in release of phosphorous to the liquor.
  6. Anaerobic digestion of the resultant sludge compared to non Bio P sludge doubled liquor phosphorus concentrations, but also significantly increased concentrations in the solid phase. Most of the phosphorus was still associated with the sludge solids.

There remain therefore two problem areas which need to be overcome before Biological Phosphorus removal can be adopted widely. These are the need for a means of maintaining performance in winter months, and the need for a means of liquor treatment which breaks the loop of returned phosphorus from sludge thickening, digestion and dewatering back to the works inlet.

There are three ways in which winter performance can be maintained.

  1. A sludge fermenter can be used to maintain volatile fatty acid concentrations, but these add VFAs in a relatively uncontrolled way, and are at odds with sewage works best practice of removing sludge from primary settlement tanks quickly and minimising soluble BOD loads. They are a moderately expensive item of capital plant. There is some evidence that over dosing of VFAs encourages mousse formation, which tends to be a feature of biological phosphorus removal plants. If sludge fermenters are to be useful therefore, a means of controlling their output to that required for P release only needs to be developed.
  2. Direct dosing of acetic acid can be used to maintain VFA concentrations, and this has the advantage of being controllable. However acetic acid is expensive and requires a high standard of storage and safety equipment.
  3. Metal salt dosing can be used to maintain phosphorus removal but if this is done a method of control needs to be established such that dosing is matched to need, and chemical removal does not take over from biological removal.

There are numerous possible techniques for avoiding the return of phosphorus with sludge liquors.

  1. If waste activated sludge is thickened quickly the phosphorus remains with the solid phase and the liquor has a quality similar to final effluent. If digestion follows and the resultant sludge is used as liquid to agricultural land no further treatment is necessary. A modest level of thickening would also be unlikely to cause problems.
  2. If dewatering is carried out following digestion a liquor will be produced containing elevated levels of Phosphorus. This stream according to experimental work represents in mass terms about 15% of the Phosphorus load to the works. This could be economically treated by a number of chemicals, such as Iron or calcium.
  3. The observation that little phosphorus is present in soluble form in the sludge liquor, implies precipitation within the digester. This is likely to be in the form of Struvite, which is observed in AW digesters. This is also undesirable as it can lead to pipework blockage. An alternative to this is to deliberately allow waste activated sludge to release Phosphorus prior to thickening and then take this liquor to a chemical removal stage.

POTENTIAL FOR PHOSPHORUS RECOVERY

Phosphorus recovery is only possible if biological phosphorus removal is carried out. This is because Iron phosphates are not suitable for processing in the phosphate industry, and also the processes used result in phosphorus being dispersed in the total sludge production. Phosphorus recovery therefore depends on the adoption of Biological Phosphorus removal. Iron salt dosing has at present been chosen because of the ease of installation and operation, however, a number of issues could result in this option becoming less attractive. The advantages and disadvantages of metal salt dosing are listed below.

 

Table 1 Advantages and disadvantages of Metal salt phosphorus removal

 

Advantages

Disadvantages

Ease of Installation

EA discouragement of chemical dosing

Inexpensive chemicals used

Sludge production increased

No liquor problems

Rising demand may increase chemical price

Reduced H2S formation in digesters

Iron and other metals dispersed in the environment

No struvite precipitation

 

Otherwise waste materials recycled to useful purpose

 
   

 

It is therefore possible to list those factors which may encourage a move to biological phosphorus removal.

  1. Regulatory requirements from the EA
  2. Closure of agricultural disposal route for metal amended sludges
  3. Closure of agricultural disposal route for all sludges and consequent increase in disposal cost/tonne.
  4. Large increase in Phosphorus removal requirements causing greater demand for metal salts.

Other factors that would encourage biological phosphorus removal are increasing ease of operation of such plants, and a value being placed on recovered phosphorus.

The potential for phosphorus recovery therefore depends in the UK and in Anglian Water particularly on the number and size of plants which are required to remove phosphate, and are potentially available for conversion to biological removal, without requiring the continuous use of iron dosing on site (i.e. without any element of trickling filters).

The list of current sites , size and process type are listed below.

Table 2 AW works requiring P removal

Site

Process

Population Equivalent

Available for Biological conversion and P recovery

Original AMPII commitment

Needham Market

Trickling Filters

11666

 

Stowmarket

Trickling Filters

19886

 

Stalham

Trickling Filters

10928

 

Halstead

Activated Sludge

11410

11410

Louth

Trickling Filters

22948

 

Brackley

Activated Sludge

20592

20592

Cotton Valley

Activated Sludge

244185

244185

Bedford

Combined AS and TF

196059

 

Chalton

Combined AS and TF

64732

 

Bocking

Trickling Filters

16767

 

Braintree

Trickling Filters

21263

 

Shenfield

Activated Sludge

46357

46357

Whilton

Trickling Filters

25513

 

Oakham

Trickling Filters

11177

 

Great Billing

Combined AS and TF

285959

285959

Broadholme

Combined AS and TF

209214

 

Corby

Two stage Biofilm/AS

133310

 
       

New commitments during AMP II

Bury St Edmunds

Trickling Filters

76471

 

Fakenham

Trickling Filters

18028

 

Dereham

Trickling Filters

13355

 
       

Totals

20

1459820

322544

     

608503

 

Great Billing is included as a plant that could potentially be used for Phosphorus recovery as the Trickling filters treat a relatively small proportion of the flow and extra Activated sludge capacity could be provided economically.

Therefore, of the 1.5 million p.e. requiring Phosphorus removal, only 300000 p.e. is potentially available for recovery immediately, with a further 300000 available relatively easily.

 

POTENTIAL PROCESS LAYOUTS FOR PHOSPHORUS RECOVERY

In order to recover Phosphorus from sewage it is necessary to identify a point in the process train where suitable conditions are likely to exist for recovery to be economic. Three likely points usually exist. These are :-

  1. The RAS return line. This is typically used in the Phostrip process where a proportion of RAS can be passed through a stripper tank, and the supernatant subjected to Lime precipitation.
  2. The waste activated sludge line. Phosphorus can be released from this sludge and then chemically precipitated in much the same way as the Phostrip process.
  3. The digester supernatant liquor line. Phosphorus released in the digesters can be precipitated from the liquors.

In order to select the most likely point for Phosphorus recovery, the advantages and disadvantages of each point need to be considered, and the possible yield of Phosphorus at each point considered. To assist with this an idealised sewage works treating 100000 p.e. is used. This works would have a phosphorus input of 300 kg/day. 20000 m3/day of sewage would be treated per day and the activated sludge plant would have a total volume of 13000m3. The RAS flow is assumed to be 20000m3/day.

This basic works is shown in figure 1. The influent Phosphorus load of 300 kg P is reduced to 20 kg in the effluent, equivalent to 1 mg/l. The remaining phosphorus is wasted at a rate of 280 kg/day.

Figure 1 P balance - Idealised Treatment works

If the case of adding a Phostrip system (Figure 2) is considered a side stream stripper tank is assumed to be added to the RAS return line taking 10% of the return sludge and stripping it. Rather idealistically it is assumed to be capable of reducing P in the stripped sludge to zero. Since sludge is recycled at 20000 m3/day, 2000m3/day of RAS will pass through the stripper each day. At twice the mixed liquor concentration this is equivalent to 31% of the sludge in the plant each day. Taking into account the other sink for phosphorus in the plant, that is sludge wastage (10% of the sludge per day), a maximum of 212 kg/day is potentially available for recovery.

 

Figure 2 Idealised P balance - Phostrip plant

The layout and balance for treating sludge liquor is shown in Figure 3. The waste activated sludge is thickened quickly to retain the phosphorus in the sludge and this passes through digestion. Partition of phosphorus in the sludge between the liquor and the solid phase is subject to very varied estimates. Some reports have indicated that a high proportion of the phosphorus becomes available in the liquid phase, however experiments at Anglian Water using trial digesters, and BNR sludges, have indicated that although elevated, sludge liquor phosphorus amounts to only 15% of the phosphorus output. The remainder is in the solid phase. This appears to indicate that insoluble products such as struvite are being formed in the digestion process. Struvite is found on AW digestion plants, typically downstream of dewatering equipment probably due to aeration stripping carbon dioxide. However there appears to be only minor problems with struvite accumulation in digesters and the quantities of solid struvite found do not amount to the total possible weight. One possible explantion for this is that under the conditions in the digester, saturation for struvite formation is reached and as cells undergo lysis, the released phosphate precipitates quickly as a sort of cell cast. These would be small enough to act as colloidal solids, probably being incorporated with organic solids.

Therefore, depending on what happens within the digester, sludge liquor treatment could yield between 40 to 240 kg of recoverable P in the effluent. Under AW conditions the lower estimate would appear to be most likely to be achieved in practice.

Figure 3 Idealised P balance - Sludge Liquor plant

The third option is to take the waste activated sludge, cause this to release P and thicken this prior to digestion, or further processes. This is possible in either anaerobic (as in Phostrip) conditions, or extended aerobic conditions. The degree of release possible will depend on the length of time that the sludge is retained for in the release tank, but could be very high.

Figure 4 Idealised P balance - Waste activated sludge plant

DISCUSSION

A number of obstacles exist to the implementation of P recovery on treatment works. These relate to the availability of suitable sites and the problem of operating a plant without a competitive process such as metal salt P removal locking up Phosphorus.

For a BNR plant concentrated streams of phosphorus liquor are available at three points. The digested liquor process would probably involve Struvite precipitation, and require a Magnesium feed to supplement high levels of Phosphorus and Ammonia normally present. For Anglian Water, this does not appear to be a good option as most of the phosphorus following digestion appears to be associated with the solid phase.

The two other options involve phosphorus recovery prior to digestion, and therefore in the relative absence of ammonia, so Calcium phosphate precipitation would be the preferred process. Both of these appear good options, but the WAS system is preferable purely for recovery. This is because assuming similar rates of P release are achieved in both cases, as this treats lower volumes of sludge daily, smaller tanks would be required, and existing dewatering equipment could be utilised for the liquor separation.

The Phostrip option would have other benefits in terms of stabilising the P removal process, by limiting phosphorus concentrations carried in the BNR sludge.

CONCLUSIONS

Practical possibilities for P recovery exist, but will depend for implementation on the economic value of the recovered material, and the need to achieve other objectives, such as widespread implementation of BNR, and possibly low phosphate sludges. All options require further investigation, and may be site specific. The process that becomes widely adopted will need to be simple, and require low manpower and maintenance.

ACKNOWLEDGEMENTS

The author acknowledges the permission of Anglian Water to present the paper. Any views expressed are those of the author, and not necessarily those of Anglian Water