Coagulation for phosphorus removal

An examination of the efficient use of coagulation for phosphorus (P) removal and energy reduction in wastewater treatment plant operation.

Increasing energy costs and carbon reduction targets continue to drive efficiencies in wastewater treatment. Optimised primary treatment plays a key role as it reduces the load to the (usually) energy-intensive aerobic treatment process and maximises the yield of high calorific value sludge for energy recovery. Enhanced primary treatment using coagulants has been shown to be effective but has not been widely implemented except where other factors, such as a lack of treatment capacity, require its use. The introduction of new discharge consents for phosphorus require coagulant chemical dosing at many sites. Correctly employed and controlled, a multi-stage dosing approach, incorporating primary treatment, offers an opportunity to not only meet the P consents but to offset some of the costs through process optimisation.

Primary treatment

 Increasing wastewater treatment operating costs combined with carbon reduction targets are driving investment in both improved efficiency and renewable energy production. In terms of overall works efficiency, primary treatment could be considered to be the most important part of the wastewater treatment process as this influences both the load requiring secondary treatment (aeration typically consumes 60 per cent of the power for wastewater treatment) and the sludge energy yield available for energy recovery (typically by anaerobic digestion).

Primary settlement uses gravity for the removal of solids. Within this process, discrete particle and flocculent settlement occurs in the upper sections of the tank whilst zone settling and compression occur as the solids concentration increases towards the tank base.

A number of options exist to improve the performance of existing primary tanks. For example, CFD modelling has been used to identify improvements in the hydraulic efficiency of primary tanks, such as through the installation of baffles which improve the conditions for self-flocculation and solids capture. Likewise, the co-settlement of crude sewage and surplus activated sludge (SAS) within primary tanks has been shown to influence flocculation and solids capture, and this has been further exploited in more novel process configurations such as the A/B process.

Enhanced primary treatment using chemical addition is another option to increase both the performance and capacity of primary tanks. The addition of coagulants (most commonly ferric salts) acts to increase the removal of both soluble and particulate contaminants in primary treatment, with solids removal typically increasing from 50-70 per cent to 80-90 per cent. Flocculants may also be used in conjunction with or as an alternative to coagulants to increase the particle size and hence the settlement rate. The cost/benefit of enhanced primary treatment options will be influenced by the:

  • Chemical cost
  • Increase in volume/calorific value of sludge
  • Reduced load to secondary treatment and associated cost saving
  • Impact on sludge production and treatment/transportation/disposal costs

Enhanced primary treatment is not widely utilised as the net benefit is not perceived to justify its implementation. It is more commonly applied where other factors necessitate its use, such as to cope with seasonal peaks from tourism, reductions in secondary treatment capacity for maintenance, or where a site has longer-term compliance issues.

New P consents

The EU Water Framework Directive (WFD) has set Environmental Quality Standards (EQS) for surface waters which will require water companies to meet more stringent consent standards for discharges from wastewater treatment works (WwTWs). In particular, many works not consented on phosphorus will now have limits imposed, with some limits potentially as low as 0.02 mg/l.

Currently, market available and economically viable phosphorus removal options are generally limited to biological or chemical removal, with the latter being much more prevalent due to the necessity for a year-round reliable carbon source to support biological treatment. Chemical treatment uses a metal salt to precipitate ortho-phosphate, with iron dosing most commonly being applied, although alum is also used.

Chemical addition can be employed for P removal before the primary tanks, into secondary biological treatment or in a dedicated tertiary treatment. To achieve consent limits of 1mg/l or under, multi-point dosing will almost certainly be required. The molar ratio of metal ion to P required for effective P removal increases as the P concentration decreases, ie the lower the target concentration of P, the greater the relative dose required.

Typical metal ion/P molar ratios for P removal
Crude wastewater Typically 1.6-1.8
Secondary treatment 2.0-2.2 in secondary treatment process
Tertiary ultra low P Up to 7 to achieve P as low as 0.1

 

This means there is significant benefit in aiming to take out the bulk of the phosphorus in primary treatment (ensuring enough P remains for the microbiological requirement in secondary treatment) as this will not only reduce the overall chemical consumption but provides the potential OPEX savings associated with enhanced primary treatment.

Site-specific data

 The optimum configuration of dosing points to minimise chemical consumption and overall operational costs will be influenced by the effluent discharge consent limit, plant configuration, dosing control strategy and the wastewater composition. Jar testing is essential to produce the data to determine the dose response curves in order to identify the optimum chemical and dosing location. This should ideally be carried out to incorporate a range of flow conditions to assess the wastewater variability.

BIOWIN model showing how the difference between primary and secondary dosing, with multi-point dosing models and different control philosophies, can be assessed. In this example, secondary dosing also provides an energy saving through reduced RAS pumping requirements
BIOWIN model showing how the difference between primary and secondary dosing, with multi-point dosing models and different control philosophies, can be assessed. In this example, secondary dosing also provides an energy saving through reduced RAS pumping requirements
  Secondary treatment energy demand (kW) Energy content in biogas (kW) Net energy gain CHP plant (kW) Net energy gain gas-to-grid plant (kW)
Without Ferric (Fe) addition

225

667

Fe addition in primary settlement tank

212

735

36.6

81.1

Fe addition in secondary settlement tank

213

700

23.6

45.3

 

Minimising cost

Process modelling is a highly effective tool to provide an overview of the impact of chemical dosing on the performance and operating costs across the works. Once the model is created, it allows a range of variables to be quickly investigated and it can be updated over time as more data becomes available. The BioWIN simulation below for a 100,000 PE works provides an example.

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  Secondary treatment energy demand

(kW)

Energy content in biogas

(kW)

Net energy gain CHP plant

(kW)

Net energy gain gas-to-grid plant

(kW)

Without Ferric addition 225 667
Fe addition in primary settlement tank 212 735 36.6 81.1
Fe addition in secondary settlement tanks 213 700 23.6 45.3

 

Conclusions

The implementation of the WFD legislation means that more sites will be reliant on chemical dosing to meet the phosphorus discharge limits. A tailored site-by-site approach offers the opportunity to not only optimise chemical consumption and ensure compliance, but to integrate this into the plants’ operation to deliver other process efficiencies. Site testing and process modelling help to identify and implement the most effective long-term solutions.

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