Delivering a Resilient Approach to Tertiary Phosphorus Removal from Wastewater

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The environmental risk associated with the discharge of phosphorus (P) from wastewater treatment plants effluent in freshwaters has been a cause for concern. In that regard,with the Water Framework Directive coming into place, stricter discharge consents below 1 mgP/L and as low as 0.1 mgP/L are being implemented. In this context, currentapproaches for phosphorus removal from wastewater effluents and, in particular, their ability to meet the very low consents are being reviewed so that it can be achieved in asustainable and economical way. The dosing of metal salts, coagulation, remains the most efficient commercially available process to remove phosphorus to these very lowlevels. However, recent events have also highlighted the limited availability of coagulants and associated to the current political situation, the increased amounts of chemical tobe required to meet new and stricter consents may lead to shortages as well as a significant increase in costs. It is then critical to optimise the coagulation-flocculation processin order to remove P from wastewater to very low levels sustainably. The efficiency of the overall process is directly affected by floc formation and their subsequent separationfrom the water. Recent research work demonstrated the significant impact water characteristics (i.e. solids, alkalinity) have on the process and emphasised the impact chemicalchoice, mixing conditions and type of floc separators have on treatment performance but also highlighted the general lack of knowledge available on coagulation in wastewaterto fully understand the mechanisms and optimise the process for all types of conditions. Two aspects are particularly highlighted: (1) the need to understand how the speciationof phosphorous impact coagulation efficacy as to meet low P standard removal of condensed and poly phosphates will be required. (2) Optimisation of the flocculationprocesses is best address with dynamic measurement of floc distributions and is impacted by sampling and transportation. It is expected that to meet effluent P concentrationsas low as 0.1 mgP/L the separation step will have to achieve suspended solids removal down to below 5 mg/L. It then becomes essential to understand these mechanisms toensure the right size precipitated aggregates are formed for each type of separator and hence minimise total costs. Consequently, the aim of the proposed project is to developa best practice guide of coagulation for tertiary phosphorus removal. To meet this aim, the project will address the following objectives:- Determine the impact of wastewater characteristics (phosphorus fractions, hardness, alkalinity, competing compounds, suspended solids) on chemical dose, mixingand separation requirements- Study and optimise mechanical and hydraulic mixing configurations for optimum floc formation including consideration for retrofit operations- Determine the connection between floc size development and separation for different solid-liquid separation processes to enable flocculation requirements to betailored to the separation processes used.- Propose a best practice guide for combined coagulation and TSR technologies for P removal to low levels