USE OF “BOX TEST” AS A TOOL FOR COMPLIANCE WITH WASTEWATER PROVISIONS OF SOCMI HON

A regional wastewater treatment plant (WWTP) in Texas is regulated by the wastewater provisions of SOCMI HON as it forms a part of the compliance strategy of the SOCMI HON dischargers. In order to comply with SOCMI HON, the WWTP has to demonstrate more than 95% biological removal across its open biological treatment system. A demonstration of compliance was accomplished using the “BOX Test” and Toxchem+ (a modeling software to evaluate emissions). The demonstration was accomplished in three phases. The first phase consists of conducting a survey of all the WWTP customers to determine their regulatory position for their waste streams, SOCMI HON compound concentration, and flow rate. The second phase was to determine biodegradation rate coefficients for the identified SOCMI HON compounds using Procedure 4, the (batch) test with (oxygen) addition (BOX), in 40 CFR Part 63, Appendix C in the regulations. The third phase was to model the biological treatment system and determine the overall biological removal using the site-specific biodegradation rate coefficients obtained in second phase in Toxchem+ model. An operating range was developed to assess compliance at various flows and temperatures with MLSS concentration being the control parameter that could be adjusted by the WWTP to achieve 95% removal of total influent load. The overall biological removal ranged from 95.03% to 99%. The influent flow rate range from 7 MGD to 20.9 MGD and temperature range between 21°C and 44°C. The WWTP achieve their compliance when their MLSS range from 2,000 mg/L to 6,000 mg/L in high flow, low temperature condition. BOX Test along with Toxchem+ modeling can be an economical option in evaluating a WWTP's biological removal efficiency and can be applied to any biological treatment facility that receives regulated discharge and needs to be in compliance with the standards.