Meeting the new low phosphorus standards

The water industry is about to face one of the biggest challenges in maintaining wastewater compliance in its history. The Water Framework Directive (WFD) had an aspiration that all aquatic ecosystems should meet ‘good status’ by 2015. In the UK, this will drive the tightening of phosphorous standards and lowering phosphorus (P) levels down to 0.5mg/l in AMP6, and then further to 0.1mg/l in AMP7.

This level of compliance will require a step change in the operational input and maintenance of wastewater treatment works. Operators will have to be up-skilled, as new technology will be used to meet these tight limits. Water utilities will also have to ensure additional capital maintenance funding is available if these new regulated outcomes are to be met.

Consents tightening

 Over the course of many AMPs, the water industry has seen Urban Waste Water Treatment Directive (UWWTD) consents tighten from 25/35 BOD/suspended solids in mg/l to then include Total Phosphorous (TP) and Total Nitrogen (TN).

With these additional consent parameters for nutrient removal, waste water treatment works have required further upgrades of the main treatment processes. For example, many water utilities have included tertiary treatment as well as metal salt dosing to achieve those phosphorous limits.

The main driver for these additional treatment stages is to achieve lower suspended solids as the precipitation of soluble phosphorous intrinsically link the removal of phosphorous to the removal of solids. Typically to meet a 1mg/l TP limit, the average solids target required is 10mg/l which is significantly lower than the previous 35mg/l target specified in the solids consent. With further tightening of TP to 0.5mg/l and 0.1mg/l P in AMP6 and AMP7; the average final effluent suspended solids targets are likely to become 5mg/l and 1mg/l respectively.

The turbidity of the final effluent at these ultra-low solids standards will be approaching drinking water standards. As an industry, we are getting closer to standards which would enable direct effluent reuse.

Technologies

Areas in the USA such as the Everglades have been subject to low phosphorous permits down to 0.05mg/l for many decades now. The type of technology used to achieve these tight standards ranges from membrane bioreactors to tertiary sand filters.

In the UK, we are moving towards the use of tertiary technologies as they are likely to present the lowest whole-life cost solutions. Two examples of such tertiary treatment technologies are described below, both originated from potable water treatment applications.

Actiflo® settling stage
Actiflo® settling stage

Actiflo® Process

The Actiflo® process consists of coagulation; flocculation; microsand ballasted settlement, ballast regeneration, and sludge production. Coagulation occurs in the first tank, where rapid mixing ensures efficient use of the chosen coagulant. Flocculation binds particles from wastewater with the microsand. The ballasted flocs have a greater specific gravity and settle in the final lamella stage.

Schematic showing Actiflo® process
Schematic showing Actiflo® process

The ballasted floc is then lifted to the hydro-cyclone where the hydroxide floc/ferric phosphate is separated from the ballast. The microsand is recovered and the flocs wasted by the centrifugal forces within the hydro-cyclone. The microsand returns to the flocculation tank, while the less dense hydroxide flocs are removed as sludge.

Comag® settling stage
Comag® settling stage

Comag®

The Comag® process is also a ballasted clarification process which in this case uses magnetite as the ballast. The first stage is coagulation tank where chemical coagulation/floc formation occurs. The wastewater and floc passes into the ballast mix tank, where recycled sludge and magnetite from the clarifier is reused and recovered magnetite from the magnetic recovery drum is returned. The ballasted flocs have a greater specific gravity and settle in a clarification stage. Settled sludge is both wasted and recycled internally.

Schematic of the Comag® process
Schematic of the Comag® process

Waste flow is sent through an inline shear mixer prior to the magnetic recovery drum where the ballast is separated from the floc. The recovered magnetite from the magnetic recovery drum drops into a slurry tank and is pumped back to the ballast tank.

Is the UK water industry ready?

 The levels of treatment described above require a step change in the operational input and maintenance of wastewater treatment works as most solutions are likely to be complex tertiary treatment technologies. Tertiary treatment is the last stage of treatment and to ensure the tertiary treatment process achieves its design intent, all the upstream process units must be fully optimised and operational. In the past this has not been critical due to relatively lax consents and a headroom available in treatment capacity.

The USA can provide a benchmark to measure the UK against: American water companies have been operating low P plants for decades. The USA also gives an insight into the level of infrastructure, maintenance and operational input required to meet low P standards.

For a typical 50,000 population wastewater treatment works, table 1 below compares the operational and maintenance input between the USA and the UK.

Operational Resources USA UK
No. of operators present on site per shift 2 Area based
24h operation presence Yes No
Training requirement Specialist General
No of maintenance engineers 2 Area based
Specialist support 1 Area based

Table 1: Operational benchmark comparison of US and UK wastewater treatment works

This highlights a clear gap in the level of operational input and maintenance. Most interestingly, the highest level of certification from the American Environment Protection Agency is required to operate wastewater treatment facilities with Low P consents.

The US benchmark may be considered to be conservative but it could be argued the UK may struggle to meet these tightened standards. Due to the level of maintenance, anecdotally it can be said that more equipment is in operation in the US which adds a high level of risk to consent failure in the UK where levels of maintenance are lower. In addition, area level specialist support such as process scientist may be over whelmed with the level of support required to maintain new standards.

An alternative solution may be to completely automate the treatment works and provide remote operation to treatment works. Artificial intelligence is already a mature technology with the water industry, with companies such as Hach and Perceptive Engineering marketing and successfully applying Real Time Wastewater Control Systems.

It is clear that the UK water industry is about to face one of the biggest challenges in maintaining wastewater compliance due to the WFD. To meet this new and stricter regulation the industry must look closely at its operation and maintenance regimes – and be prepared to make significant changes.

 

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