Water & Wastewater Treatment

In the know Technically speaking: jar testing The test revealed the amount of phosphorus present a er a range of ferric doses was applied 36 | OCTOBER 2015 | WWT | www.wwtonline.co.uk 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. Diurnal profiling. Phosphorus levels varied widely at the plant according to the time of day 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 (see graph, top leŠ). 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 (see graph, below leŠ), 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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