Water & Wastewater Treatment Magazine
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22 | FEBRUARY 2019 | WWT | www.wwtonline.co.uk The Knowledge Wastewater treatment phosphorus concentration de- creases i.e. the lower the target concentration of phosphorus the greater the relative dose. This means there is signifi- cant benefit in aiming to take out the bulk of the phosphorus in primary treatment (ensuring enough phosphorus remains for the microbiological require- ment in secondary treatment) as this will not only reduce the overall chemical consump- tion but provides the potential operational savings associated with enhanced primary treat- ment. Any excess metal salts added to the sewage, due to the overdosing required to achieve low effluent P concentrations will react to remove alkalinity from the sewage. Alkalinity is required for ammonia treatment and if the alkalinity concentration is too low the final effluent am- monia discharge consent may be breached. A number of unknowns need investigating prior to progressing low phospho- rus capital schemes. These include: 1) what is the molar ratio of metal ion to phos- phorus to dose; 2) what is the best chemical for the plant – e.g. Ferric Sulphate, Ferric Chloride, Poly Aluminium Chloride (PAC), Aluminium sulphate, rare earth metals or a combination of metal salts and polymer; 3) whether alkalinity dosing is required; and 4) whether alternative technologies, such as sono- electrochemical treatment, magnetite or algae reactors are viable or more economic. 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. Biological P removal Biological Phosphorus removal can be achieved in an activated sludge plant by incorporating an anaerobic zone and where there are conditions in the influ- ent to the secondary treatment comprising adequate short chain volatile fatty acids (SCV- FA) such as acetic, propionic, and butyric acid, suitable for the phosphorus to be absorbed into the sludge. However, where the influent does not contain adequate SCVFA, it is still pos- sible to achieve treatment by adding additional anaerobic zone capacity to enable the hy- drolysis of particulate organics to provide a carbon source. Once biological phospho- rus conditions are achieved, activated sludge can store polyphosphate in the range of 3-6% phosphorus by dry weight as compared to 1-1.5% assimi- lated in a conventional activated sludge plant. In the UK the phosphorus rich sludge from biological P treatment is then generally re- moved from the activated sludge plant and transferred for treat- ment in an anaerobic digestion process. Usually more than 50% of the phosphorus in the sludge is released in the digestion process and tends to be recycled in the return liquors to the inlet of the sewage treatment works. This recycle stream needs to be considered in the design. These are the unknowns which will need investigating prior to progressing to a capital scheme: 1) Is the crude sewage suitable for Biological Phospho- rus (BioP) removal (sufficient SCVFA or hydrolysable solids)? 2) Does the crude sewage char- acteristics change over the sea- sons? 3) Is an additional carbon source required? 4) Does the settlement of the sewage change with BioP?; 5)Does the sludge make up change and what are the nutrient characteristics of the sludge? 6) If BioP is applied, then how much phosphorus is released again a›er diges- tion/advanced digestion to be recycled to the inlet works? 7) What are the potential impacts of elevated phosphorus in the sludge handling assets i.e. struvite precipitation? 8) What technologies are available to capture the phosphorus in the recycle stream and how reliable and efficient are they? In order to understand the unknowns above, data, model- ling and investigation may be required. This could include sampling and analysis of crude sewage over a period of 1 to 3 years to gather data on total phosphorus and orthophos- phorus; COD fractionation to quantify soluble readily biodegradable COD (rbCOD) and to understand the degree of particulate COD hydrolysis required; analysis of short chain VFA (SCVFA) as a fraction of soluble readily biodegradable COD; and diurnal profile meas- urement. From the sample data above, modelling so›ware simulations can be run to determine if a VFA fermenter is required. Laborato- ry scale anaerobic digester trials (possibly including advanced digestion such as thermal hydrolysis upstream of diges- tion) can be carried out with dewatering of the digestate and analysis of the P concentration in the return liquors. Investiga- tion of P removal processes on the return liquors, and bench scale ASP for BioP modelling, can also be used. A new service Aqua Enviro in parnership with EnviroSim (the devel- opers of BioWin process modelling so›ware) are able to provide a holistic assessment to least cost P compliance, in- corporating the considerations above. This service includes detailed site assessment, sam- pling and analysis to include full wastewater fractionation. BioWin models are then devel- oped to consider the changes required, taking into account the interactions between the wastewater and sludge assets. The optimum balance between P removal (chemical and bio- logical) and recovery (i.e. as struvite) can be understood. The process modelling (Bio- Win) can be used to investigate a host of factors which affect on the design of the process. In order to design the most economically viable phospho- rus removal process, designers require information about the sewage composition, reactions with metal ions and possibly mathematical modelling to provide confidence for capital expenditure in AMP7. Now is the time to start these investi- gations. Above: The effect of ferric dosing on phosphorus A schematic of the BioWin modelling process. Crude Wastewater – typically 1.6 to 1.8 Secondary Treatment - 2.0 to 2.2 Tertiary / Ultra Low P - up to 7 to achieve P as low as 0.1 TYPICAL METAL ION/P MOLAR RATIOS FOR P REMOVAL