Filter Backwash Recycling and a Cryptosporidium Concern
A 20 MGD conventional plant with six dual-media filters recycles filter backwash water to the head of the plant at a rate of about 1.2 MGD, roughly 6 percent of plant flow. The recycle stream is high in turbidity and particles and is returned just ahead of coagulation. The plant's source water is a surface supply with a history of low-level Cryptosporidium detections, and the state has classified it as bin 2 under the LT2 rule, requiring a microbial toolbox approach. The plant currently uses conventional filtration with no additional Cryptosporidium barrier beyond filtration and free chlorine. Free chlorine is known to be a weak Cryptosporidium disinfectant. The plant has an online particle counter on combined filter effluent and a turbidimeter on each filter. Recent particle counts on combined effluent have risen slightly, and the operator suspects the recycle stream is contributing to filter loading and particle passage. The utility is considering installing UV disinfection, but budget is tight and the state wants a plan.
1. How should the operator evaluate whether the recycle stream is a meaningful risk factor for Cryptosporidium, and what should be done about it?
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The operator should treat recycle as a potential concentration point: backwash water captures particles that passed through or were captured by filters, and returning it ahead of coagulation can increase particulate loading and, if any oocysts are present, reintroduce them. The evaluation should include mass balance and monitoring: measure recycle turbidity, particle counts, and flow, and compare plant performance with recycle on and off. The slight rise in combined effluent particle counts is a signal worth investigating, but it could also come from raw water change or filter condition.
The defensible action is to manage recycle rather than simply stop it, since stopping may create water supply and discharge issues. Options include returning recycle at a controlled, low rate, returning it to a point after coagulation or to a dedicated treatment step, or treating recycle (for example with UV or filtration) before return. The operator should document the recycle rate, the monitoring data, and the rationale. The reasoning is that recycle is a recognized Cryptosporidium risk factor in the toolbox, and the utility should be able to show it evaluated and controlled that risk.
2. The utility is considering UV. How should the operator frame UV's role relative to the existing free chlorine and filtration?
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UV is attractive because it inactivates Cryptosporidium at low doses, whereas free chlorine is a poor Cryptosporidium disinfectant. The operator should frame UV as a multi-barrier addition, not a replacement for filtration or chlorine. Filtration removes particles and some oocysts; chlorine provides a residual and bacterial and viral control; UV adds a Cryptosporidium barrier. The state's bin 2 classification means the utility needs enough toolbox credits, and UV can supply them.
The operator should also consider practical requirements: UV needs low-turbidity water to be effective (transmittance matters), so it is typically placed after filtration. It requires power, lamp maintenance, and validation. The operator should recommend a design basis and a monitoring plan, and should not assume UV solves everything, since it provides no residual. The judgement is to integrate UV into the existing barrier concept and document the credits it provides.
3. What monitoring and process control changes should accompany a decision to keep recycling at the current rate while UV is evaluated?
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If recycling continues at the current rate, the operator should tighten monitoring: track recycle flow and turbidity continuously, watch combined effluent particle counts and individual filter turbidity, and correlate changes with recycle events. The operator should also verify coagulation is optimized for the higher particulate load and consider returning recycle at a steadier rate rather than in slugs. A slug of high-turbidity recycle can upset filters, so equalization or a controlled return rate is preferable.
The operator should document the monitoring plan and the decision rationale for the state, including why recycling continues pending UV. The reasoning is that interim control should be data-driven: if particle counts or turbidity rise with recycle, the rate or return point should change. The operator should also keep the Cryptosporidium risk assessment current and be ready to adjust if source water detections increase.