Current EU legislation has led to the implementation of new environmental quality standards in relation to specific trace contaminants in surface waters. This has created a focus on the reduction of trace substances resulting from point sources, including industry and municipal WwTW effluents.
There are two main strands to this. The first is the UKWIR chemical investigation programme (CIP), which started in 2009 by determining the concentration range of trace pollutants in WwTW effluents throughout the UK; the CIP has now progressed to a more focused approach, looking at specific site discharges and evaluating the environmental risk using a catchment-based approach. This has resulted in the implementation of tighter consents on a variety of parameters.
The second factor at play is that new terms and consent limits for total phosphorus on municipal WwTW discharges are being implemented across the UK under the Water Framework Directive (WFD). WFD also uses a catchment-based approach, and employs the use of modelling to target specific works outlets, often applying tighter consent limits to meet new environmental quality standards.
Phosphorus (P) is an essential nutrient for the growth and metabolic reactions of all plants and animals, and its effect on watercourses is well understood. Within a watercourse, phosphorus concentration has a direct effect on ecological biodiversity. Increased phosporus concentration favours algal growth which in turn reduce macrophyte diversity, causing stagnant environments, resulting in the depletion of dissolved oxygen in water through eutrophication.
The same level of understanding has not yet been achieved concerning CIP compounds such as steroid hormones and pharmaceuticals; however, this does not mean that such compounds may not have profound effects.
Dosing to eliminating trace pollutants
Recent changes in consents have resulted in many new assets being installed to remove phosphorus, highlighting the need for wastewater treatment processes to be optimised to reduce specific trace substances in effluent discharge.
The most basic form of treatment available for trace substances is chemical precipitation and settlement. This can take place at various stages of the wastewater treatment process and can be enhanced by optimising factors including contact time, mixing rate and chemical dosing. The advantages of chemical dosing include improved settlement rate, greater removal of colloidal BOD and potential for enhanced precipitation of trace substances within the settled fraction. When all of these factors are applied to primary stages of treatment, secondary and tertiary treatment capacity is increased. This creates potential for asset reuse in future schemes, reducing both CAPEX and OPEX.
Dosing using metal salts is the most cost effective and widely used method of chemical precipitation. Metal salts aid coagulation by neutralizing negative charges on suspended and colloidal matter, to form compact flocs suitable for greater levels of removal by settlement.
Metal salt dosing for phosphorus removal allows the precipitation of phosphorus through the formation of metal phosphate compounds, which are then combined within metal hydroxide flocs and finally removed via settlement. Adsorption and precipitation of CIP compounds with metal hydroxide precipitates has not been fully quantified as yet.
Dosing is not without risk, and must be optimised to ensure that effective treatment is achieved and excess dosing chemicals do not themselves cause final effluent discharge consents failures. Alkalinity must also be carefully monitored to prevent loss of process performance in the secondary stages of treatment.
There is no available literature on the effect of “conventional” chemical dosing on CIP compounds such as certain pharmaceuticals and pesticides. However, as chemical dosing prior to settlement can increase the removal of inorganic and organic colloidal particles, this may well improve settlement properties or degradation rates of these specific CIP compounds.
The value of jar testing
Jar testing is a vital technique for improving plant performance by optimising aspects of treatment processes in relation to mixing, contact time, flocculation, settlement rate/time, precipitation and chemical dosing. It can provide valuable insight for the person responsible for plant optimisation and asset management.
This type of testing is undertaken utilising specific jar testing apparatus, where the mixing rate, contact time and settlement period can be controlled, to mimic specific on-site processes such as the primary settlement of crude sewage.
To ensure operating costs are kept to a minimum, the level of treatment achievable and quantity of chemical required must be investigated prior to installation to undertake a cost benefit analysis.
In combination with routine analysis of the settled sludge and supernatant liquid fraction, response curves can be created illustrating the effects of the varying operation parameters on the settlement process in question.
Aqua Enviro undertakes independent jar testing studies to mimic the chemical dosing of on-site primary, secondary and tertiary settlement processes. These can investigate the effect of a range of chemical doses on the relevant wastewater matrix, to identify the most effective treatment, quantity/concentration of chemical required and any resulting effects such as reduced alkalinity.
A case study on phosphorus removal
One recent study was commissioned by a water utility aimed to identify the optimum chemical, dose and location where the greatest reduction in phosphorus could be achieved. Jar tests were undertaken using crude sewage from a municipal WwTW at a range of ferric doses. Once the mixing and settlement period was complete, the resulting concentrations of all forms of phosphorus were quantified.
This chart shows that the majority of soluble phosphorus in wastewater is in the form of PO43–P. Particulate (insoluble) phosphate accounts for around 30% of the Total phosphorus load.
At this site the most effective dose for the removal of all forms of phosphorus was 30mg/l as iron. Other key findings from this study included the effect of overdosing, where it was established that dosing in excess of 30 mg/l caused elevated levels of total phosphorus in the final effluent, as a result of ferric hydroxide floc carryover.
The spikes observed in the diurnal profile of influent PO43–P from the same site show that iron dosing must be controlled in a real time environment to provide the right dose at the correct time, to make optimum use of the metal salts to precipitate available phosphorus and prevent iron carry over.
By utilising jar testing and diurnal profiling as a means to investigate the effect of chemical dosing on primary, secondary or tertiary settlement, greater levels of trace substance removal may be achieved to help meet new or current final effluent discharge consents.
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