Use of a mass balance model for developing guidelines for treatment plant recycle streams

Recycling treatment, plant waste streams has become an important issue and the EPA is currently developing a rule for controlling and potentially limiting these streams. The impact to the overall treatment process and the relative risk of various recycle streams can be evaluated with a materials balance model of a treatment plant. A steady-state materials balance model was developed and applied to the recycle of backwash waste water, clarifier sludge supernatant and sludge dewatering supernatant. Recycling backwash water reduced the plant treatment effectiveness from 3.0 log to 2.95 log removal when the recycle stream was treated (0.5 log removal) and 2.84 log removal when not treated. Recycling clarifier sludge supernatant resulted in a reduction of performance from 3.0 log to 2.78 log removal with adequate treatment (0.5 log removal) and 1.95 log removal with inadequate treatment (10% removal). The model was used to identify vulnerable treatment conditions. Conventional treatment with a poorly operated or upset clarifier was identified as a significant risk with the overall treatment effectiveness decreasing from 3.0 to 2.3 log removal.