Full scale optimisation of sludge dewatering and phosphate removal at Harnaschpolder wwtp (The Hague, NL)

At Harnaschpolder wwtp phosphate removal from wastewater and sludge dewatering is optimised by dosing magnesium hydroxide to digested sludge. To optimise sludge dewatering and phosphate removal, a full-scale test has been executed since August 2015 using temporary equipment. In this test magnesium hydroxide is added to the digested sludge buffer. Significant positive results are achieved. An excellent quality of the reject water is obtained, through which the phosphorus load in this reject water returned to head of works is negligible. This decrease in combination with other optimisations are beneficial for the biological phosphorus removal process. In 2016 the phosphorus effluent quality remained constant, while 42% less iron chloride was added to meet the legal phosphorus effluent quality requirements (yearly moving average Ptotal < 1.0 mg P/l). The polymer usage decreased by 25% and the year average dry matter content of the dewatered sludge increased from 22% in 2014 to 24% in 2016. Struvite scaling or blockage is avoided in piping, pumps or dewatering equipment, through optimised control of dosage of magnesium hydroxide. The full-scale test results prove that the addition of magnesium hydroxide to digested sludge leads to a significant cost reduction and major environmental benefits. In 2017 a permanent installation will be realised. The magnesium hydroxide dosage will be further optimised based on lessons learnt. Also an improved operation control of sludge dewatering is possible through additional in-line measurements for dry solids content of dewatered sludge. It is therefore expected that results will further improve concerning the use of iron chloride and polymers, dry solids content of dewatered sludge and phosphate effluent quality. Through this optimisation, the operating costs1 of Harnaschpolder wwtp are reduced by over 4%. The return on investment is estimated at 1.5 years.