Struvite precipitation in the sludge stream at slough wastewater treatment plant and opportunities for phosphorus recovery

Struvite precipitation has been a problem at Slough wastewater treatment works for a number of years. The plant has recently been extended with a 3-stage Bardenpho plant which has reduced the phosphorus concentration in the effluent and led to an increase in struvite precipitation in the digested sludge pipelines. These problems have been overcome in the short term by dissolving the struvite in 10% sulphuric acid. It is hoped to provide longer term protection against struvite precipitation by installing magnetic water softening devices at key locations, This form of protection is already providing some benefit at one point on the site. The phosphorus concentration and loads at a number of points on the site have been investigated. By combining two streams, the centrate from the sludge dewatering plant and the liquor from thickening the waste activated sludge, a relatively clean stream containing 160 mgl(-1) of soluble phosphate, a load of 136 kgd(-1) can be produced. In order to make best use of this stream it is important to prevent struvite formation at other points in the plant where the precipitation can cause operational problems.