Avoiding Struvite scaling in sludge treatment of a WWTP

The problem

Struvite scaling is a well-known problem resulting in stoppages and mechanical breakdown. The source of struvite is the presence of phosphate ions at the entrance of the sludge treatment units of the WWTP. With the start-up of a thermophilic sludge treatment unit’ severe scaling in the heat exchangers and the feed tank of the centrifuge appeared.

The solution
Injection of Ferric Chloride and Sodium Aluminate at various steps of the cascade of thermophilic reactors.

The rationale is that the injection of Ferric Chloride and Sodium Aluminate reduces the concentration of phosphate below the solubility level of struvite by precipitating non scaling salts that are removed with the digester sludge.
The plant has been operating for the last year without problem of struvite clogging. This was achieved by monitoring the concentration of phosphate in the various flows and controlling the flow rate of the chemical addition.
Improvement of the injection system of the chemicals increased the efficiency of the phosphate removal resulting in chemical cost savings. The use of Sodium Aluminate also increases the alkalinity in the reactors.

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