Three years experience of operating and selling recovered struvite from full-scale plant

The adoption of phosphorus removal at sewage treatment works (STW) creates two main problems. Firstly large amounts of sludge are produced and secondly the quantity of the effluent deteriorates due to the increase in the phosphorus load of the sidestream. Furthermore, these processes do not remove phosphorus in a form that would enable it to be recycled. Therefore in order to control these process difficulties and produce a recyclable phosphorus product a sidestream struvite crystallisation reactor was developed. The struvite was produced in a fluidised bed reactor using dewatered filtrate from anaerobic sludge digestion. Magnesium hydroxide was added in a magnesium to phosphate ratio of 1:1 and the pH was adjusted to between 8.2-8.8 with the addition of sodium hydroxide. A retention time of 10 days alowed the growth of pellets between 0.5-1.0 mm in size. The recovered struvite contained only minute traces of toxic substances and was sold to fertiliser companies for 27,000 yen tonne(-1). It is used to enhance existing fertilisers, which are widely used on paddy rice, vegetables and flowers.