* Two locations ended Laundry operations late in FY 1995 so that the number of Laundry sampling locations for FY 1996 was eight.

Our goal was to generate accurate estimates of wastewater discharge flows for all identified sampling locations. For accurate discharge flow estimates, we used flow rates from wastewater flow meters installed in 39 sampling locations as shown in the Questionnaire responses. For unmatured sampling locations in clinical, research, or other laboratory facilities, we decided to use a discharge flow model.

To obtain input for this model for each sampling location, we asked in the Questionnaire for:

1. Numbers of connected fixtures (i.e., laboratory sinks and fume hood (cup) sinks),
2. Numbers of connected glass washers,
3. Numbers of connected photoprocessors, and
4. Estimated flow contributions from any water treatment systems discharges,² non-contact cooling water discharges³, and sanitary wastewater discharges.
5. Sizes of final pH adjustment or limestone chip tanks.

For Questionnaires with incomplete responses and for all non-respondents, we used discharge flow estimates reported by the facilities that appeared in MWRA permitting and inspection records.

The model we used for discharge flow estimating was based upon a unit fixture flow rate and upon indicated numbers of laboratory sinks and fume hood (cup) sinks in returned Questionnaires. We initially intended to use a unit laboratory fixture flow rate of 10 gallons per hour that is the applicable design rate specified in the Massachusetts Plumbing Code⁴. However, we decided to perform a back-calculation of unit flow rates when we received 14 returned Questionnaires that included both wastewater flow meter values and responses for items 1.- 4. above. The number of fixtures for these 14 sampling locations varied from five fixtures to 1,000 fixtures. Of the 14 back-calculated unit fixture flow rates, eight (or 53 percent) were between 3.0 and 7.0 gallons per hour assuming an eight-hour operating day.

We found good conformance to the 14 reported flow meter readings when we used a unit flow rate of 10 gallons per hour each up to 30 fixtures and 5 gallons per hour each in excess of 30 fixtures for eight hours per day. In addition, we arbitrarily used 100 gpd as the estimated daily discharge flow from glass washers and 60 gpd from photoprocessors (based upon a rinse water replenishment rate of 0.5 gallons per minute for two hours of use per day). Therefore, the model we used to estimate discharge flow was as follows:

Since we made about 140 discharge flow estimates (about 40 percent of the sampling locations studied) using this model, the validity of the model was important. For the 14 sampling locations that responded with both metered discharge flow rates and fixture count information, we found that the flow estimates produced by the model equation were less than 15 percent greater on average than the metered values. Therefore, we were satisfied that the model had validity and would produce sufficiently accurate discharge flow estimates for this study.

For flow estimates based only upon neutralization vessel or limestone chip tank size, we assumed an eight-hour operating day and a 30 minute wastewater residence time in the tank. Since most of the returned Questionnaires included fixture count information and the neutralization tank assumptions were quite rough, we were pleased that only five discharge flow estimates had to be made using this method.

Table A, entitled Wastewater Flow: Questionnaire Data and Calculations, appearing in Appendix A, shows the data from returned questionnaires as a spreadsheet, the laboratory flow model results, and the final selected flow estimate for each sampling location. For each study group, the table shows that total estimated flows were approximately as follows:

* Two locations ended Laundry operations late in FY 1995 so that the estimated total Laundry discharge flow for FY 1996 was about 68,800 gpd.

We assumed that these estimated flows were constant over the entire FY 1995 and FY 1996 study period. To confirm the Questionnaire responses and model results, we reviewed discharge flow estimates reported by the facilities that appeared in MWRA permitting and inspection records. Our review showed that the reported discharge flow estimates exceeded those of the model for about two-thirds of the facilities. This was an expected result since it is evidently common practice for facilities to overestimate their discharge flows for discharge permitting purposes. While our model produced lower flow estimates in many cases and thus lower mercury loadings estimates, we believe that the overall results of these estimates are valid for each group of facilities.

In the 1992 Local Discharge Limits Development Report prepared for the MWRA, PEER Consultants had estimated that the discharge flow from all permitted industrial discharges was about 12.6 million gallons per day (MGD). While PEER examined flow data from an earlier period and we used a different method to estimate discharge flows, it is of interest that our estimated discharge flow of 2,055,000 gpd (or 2.05 MGD) for all five study groups may represent only about 16 percent of the total permitted industrial discharge flow to the MWRA Metropolitan Boston sewer system.

        ²It should be noted that MWRA regulations - 360 CMR 10.023 (19) - require that any water treatment filter backwashes be specifically authorized and meet all regulated limits and prohibitions.

        ³It should also be noted that discharges of non-contact cooling water to the MWRA sewer system are prohibited (with some exceptions) per MWRA regulations - 360 CMR 10.006 (2) and 10.023 (2).

        ⁴Massachusetts regulations 248 CMR 2.13.

 

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