Optimal coagulant dosing in primary treatment of WWTP Auckland, New Zealand

The question from WaterCare, New Zealand

Our customer WaterCare treats the water and wastewater of Auckland, New Zealand. Their grit tanks (GTs) and primary sedimentation tanks (PSTs) were undergoing a retrofit. Coagulant was going to be dosed. The two main questions were:

1. What is the optimal dosing point for the coagulant, as such that good mixing with the water takes place?
2. Can we minimise the risk on shortcircuiting?

We have run different CFD simulations, at different flow rates, to investigate where the best coagulant dosing location was. This was hard to determine without CFD... Additionally, we investigated the shear distributions along the channels, allowing coagulation to take place. We screened the system for general improvements (e.g. baffle changes, etc).

We accounted for the aeration in the PSTs, by including the air as a second phase. We have proven this approach many times before at full-scale plants. It is highly accurate. The PSTs had no aeration, but both GTs and PSTs were part of one large simulation for optimal accuracy.

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As seen in the below figure, the real geometry was taken into account, to realistically predict the flow. Two different flow rates were simulated to assess system performance at normal and peak conditions.

The importance of optimal dosing is illustrated in the below figure. The colour scale indicates the local coagulant concentration. In such straight channels, if not dosed well, coagulant can flow downstream, rather concentrated without mixing. We used CFD to optimise the coagulant mixing.