Marine Sciences Investigations in Support of Level Oftreatment Decisions for Municipal Wastewater Discharges in Puerto Rico

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ABSTRACT

The Puerto Rico Aqueduct and Sewer Authority (PRASA) currently has National Pollutant Discharge Elimination System (NPDES) permits that incorporate relaxation (waiver) of BOD and TSS secondary treatment requirements for six of its regional wastewater treatment plants. The waivers are available under Section 301(h) of the Clean Water Act. Maintaining the waivers requires, in part, continuing and detailed evaluation and documentation of effluent quality and the effect of the discharges on the marine environment. Monitoring has included effluent quality, receiving water quality, sediment quality, fish tissue bioaccumulation, and community-level monitoring to compare fish, benthic invertebrate, and phytoplankton communities found within ocean outfall mixing zones with those at control sites to assess whether 'balanced indigenous populations' are being maintained. Physical oceanographic monitoring has assessed ambient current velocity and direction, water column stratification, and effluent plume diffusion and dispersion. In a separate related study, it was found that a high definition fathometer could be used to identify the real-time location of the effluent plumes. The intent is to use this new methodology to better define and focus the water quality sampling and at the same time, reduce the overall cost of the monitoring program.

None of the studies have indicated adverse effects on the marine environment as a result of the advanced primary discharges. PRASA is currently resubmitting NPDES and 301(h) waiver applications for three of the facilities as their current NPDES permit terms expire. EPA has already announced its intent to maintain the waivers for each of the other three facilities, stating that '...the current level of treatment...is protecting the environment and public health.' This presentation summarizes the treatment plant and receiving water data upon which the EPA conclusions were based.

KEYWORDS

301(h) waiver, water quality standards, BOD, TSS, ocean outfall, diffuser, balanced indigenous population.

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