Department of Utilities

Pilot Plant

Chesterfield County currently employs the Maximum Allowable Headworks Loading Method (MAHL1) in order to calculate its pollutant discharge limits. Under this method data about Proctors Creek is combined with federally assigned pollutant limits as outlined by the Clean Water Act. Domestic contributions of wastewater are subtracted, and the remaining available capacity for a particular pollutant is divided among the number of Industrial Users (IUs) within the county. Although simple, this method fails to accurately depict the true loading capacity for a certain pollutant at the local level. A pilot plant was created to ensure that pollutant limits for Chesterfield County are site specific for Proctors Creek. The pilot plant accurately models the wastewater treatment process at Proctors Creek, but on a scale of 140,000 to 1 (see pilot plant diagram below). Ongoing metal inhibition studies offer the possibility of true site specific pollutant discharge limits for local area industries. Preliminary studies with zinc, nickel, and lead have generated very promising results. A slug load discharged into the sanitary system can be modeled by spiking the pilot plant with a specific concentration of metallic pollutant for a period of 14 hours. Both positive correlations between nickel and zinc influent concentrations with respect to effluent ammonia concentrations were realized. Lead appeared to be detrimental to the biological component of the treatment process, producing a negative relationship between lead influent concentration and ammonia effluent concentration. Current steps are being taken to broaden this initial study to include other metals such as copper and silver. There are significant advantages to the outcomes of this inhibition study. First, the Maximum Allowable Industrial Load (MAIL2) can be adjusted to match the physical limits of Proctors Creek for pollutant removal, which preliminary data suggests dramatically exceeds that of current limits. Second, more legally credible local limits can be established. Lastly, the outcomes of the pilot study will result in a significant cost savings for industries and governmental bodies alike. Industrial cost savings will be due to a reduction in the pretreatment technology necessary to maintain current local limits. Reduced sampling frequency and pretreatment man-hours are potential benefits that Chesterfield County will experience given the success and implementation of pilot study results.

Pilot Plant Diagram

1 Maximum Allowable Headworks Loading Method (MAHL) Pollutant by pollutant, treatment plant data are used to calculate removal efficiencies, before applying the most stringent criteria (i.e., water quality, sludge quality, NPDES permit, or pollutant inhibition levels) to back calculate the MAHLs. Subtracting out contributions from domestic sources, the available industrial loading is then either evenly distributed among the IUs, or allocated on an as needed basis to those IUs discharging the pollutant above background levels.

2 Maximum Allowable Industrial Load (MAIL) The MAIL is the total daily mass that a POTW can accept from all permitted IUs and ensure the POTW is protecting against pass through and interference.

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