Application: Assessment of the lime, coagulant and polyelectrolyte dosing performance at Ogston Water Treatment Works.

Customer: Severn Trent Water

Service: Audit of chemical dosing and mixing arrangements and CFD of clarifiers.

Clarifier

Problem: Severn Trent was investigating the feasibility of refurbishing the old works at Ogston increasing output from the current 25MLD to 40MLD. Whilst hydraulically designed to achieve this, it was believed that operating the plant at maximum capacity would be undesirable due to the poor performance of the clarification process. Problems identified with the clarification process included inadequate mixing of ferric coagulant, the need for lime addition and the dosing position of lime and polyelectrolyte and too short a contact time for the formation of microflocs.

Objectives: To make an assessment of the current mixing performance for each of the chemicals dosed and to identify opportunities for mixing performance improvement. To recommend improved dosing and mixing arrangements, which would conform to best practice and possibly achieve chemical cost savings.

Polyelectrolyte Dosing - Poor Mixing

Approach: A site audit at Ogston was undertaken to collate information on the existing chemical dosing and mixing arrangements upstream of the clarifiers. The subsequent review considered the mixture quality for each chemical addition point and made recommendations for process improvements. All calculations were based on correlations taken from BHRSolution's Water and Wastewater Mixing (WWM) Design Guide. An assessment of the flow distribution to the clarifiers was also carried out.

Build up of ferric scum in distribution channel is evidence of poor mixing

Outcome/Benefits: The review concluded that the existing dosing and mixing arrangements were inadequate, resulting in poor mixing and floc growth contributing to the sub-standard performance of the clarifiers. It was also found that the inlet flow channel to the clarifiers was poorly designed, leading to non-uniform loading and variable performance of the clarifiers. Recommendations were made to relocate the coagulant and polyelectrolyte dosing positions. Suggestions for optimising mixture quality included the installation of a static mixer, improvements in additive distribution and a reassessment of chemical dose rates.

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