Optimisation of anaerobic sludge treatment from enhanced biological phosphate removal by Airprex® Struvite Precipitation System
Langereis, R. and Geraats, B., SH+E, Netherlands
(free)
In WwTP with biological phosphate elimination, high phosphate concentrations occur due to the stored polyphosphate, being released from bacterial cells during digestion. The presence of high levels of phosphate may lead to a number of negative consequences, such as formation of Struvite deposits in the pipework, significant impairment of sludge dehydration and the need for higher polymer dosing. A major fraction of this phosphate load is returned to the aerobic step of the wastewater treatment causing major financial and operational problems – these drawbacks can be overcome using Airprex®. The Airprex® was developed initially at WwTP Berlin Waßmansdorf (1.2 M p.e.) and is successfully operating on two other large sites. The system utilises air stripping and dosing of MgCl2, resulting in controlled crystal formation and recovery of this re-saleable mineral fertiliser. Based on the results to date, biological P elimination combined with AirPrex® is the more cost-effective method of biological P elimination. Further details will be provided of the installation at Amsterdam WWTW (1.7M p.e.).
Keywords: Struvite, Phosphate recovery
Introduction
Applying biological phosphate removal in waste water treatment plants is a sustainable and cost efficient way of reducing the phosphate load in wastewaters. In biological phosphate removal plants, the chemical consumption and the accompanying chemical sludge production is reduced. The technology is well established and has been applied at large waste water treatment plants for the cities of Berlin, Mönchen-Gladbach in Germany and Echten in Netherlands and with a number under construction.
In sewage treatment plants with biological phosphate removal, high phosphate concentrations occur in the digester’s anaerobic environment, due to the stored polyphosphate being released during digestion. Also the breakdown in the digester of other organic phosphate components increases the phosphate level in the sludge. The (dissolved) phosphate stabilises the strongly hydrophilic colloidal system which is present in the sludge, thus seriously impairing sludge dehydration. In the presence of phosphate, water is bound to proteins, EPS, etc.
Depending on the pH value, undesirable struvite deposits are formed and scaling occurs in the pipework systems of the digesters and dewatering units. This results in significant operational costs. Lowering the phosphate concentration immediately downstream from the digesters, eliminates the occurrence of these deposits and lowers the operational costs. Also, it has been demonstrated that lowering the phosphate concentration of the digested sludge, prior to dewatering, significantly improves the dewatering and increases the drysolids concentration of the dewatered sludge with equal or lower polymer costs.
Unfortunately the biological stage of the waste water treatment plant is recharged with high phosphate loads from the sludge liquor, which results in considerable additional operating costs for a sewage treatment plant.
In the sludge line of bio-P over-saturated solutions of ammonia and phosphate salts are found. These salts precipitate (as struvite) when the CO2 is stripped out by change of temperature or pressure. Ammonia, normally is present in sufficiently high concentrations. The formation of stuvite strongly depends on the bicarbonate buffer capacity, pH, magnesium concentration and phosphate concentration.
