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Confirming BAF Performance for Treatment of CEPT Effluent on a Space Constrained Site

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The City of San Diego (City) operates the Point Loma Wastewater Treatment Plant (PLWTP) which can nominally treat and produce up to 240 million gallons per day (mgd) of chemically enhanced primary treated (CEPT) effluent for ocean disposal. The City is interested in understanding the performance capabilities of Biological Aerated Filters (BAFs) in treating CEPT effluent to secondary treatment standards. Lack of performance data in warm climates and the need to evaluate the BAF technology under local conditions and wastewater quality were primary reasons the City invested in a pilot test. Two leading manufacturers with BAF units currently operating at greater than 30 mgd capacity were asked to propose units that could meet the anticipated full scale operating conditions and effluent quality limits.

The pilot testing confirmed that the BAF technology is capable of producing secondary treated effluent that meets anticipated discharge requirements. Both units were able to meet the discharge requirements during simulated wet weather and dry weather conditions.

The presence of nitrifiers and the corresponding 5-day nitrogen oxygen demand (NOD5) imparted during the TBOD5 analysis were found to correlate with the amount of TSS in the effluent. The results also indicate that meeting the 30-day secondary treatment standard for TSS of 30 mg/L may not equate to meeting the TBOD5 30-day average limit of 30 mg/L. For both pilot units, the effluent TBOD5 exceeded this limit when TSS reached 30 mg/L. Meanwhile, the CBOD5 concentration is shown to be 10 to 15 mg/L less than the 30-day average limit for CBOD5 of 25 mg/L. A permit limit negotiated in terms of CBOD5 is thus desirable.

Solids yields (lb TSS produced per lb TBOD5 removed) were higher than initially expected for both BAF pilot units (i.e., > 1 lb TSS per lb of TBOD5 removed). The solids were particularly high in Phase II of the testing when the solids yields for the Biostyr and Biofor-C were calculated to be 1.21 and 1.15, respectively. The difference in yields of the two BAFs is believed to be related to the differences in backwashing procedures.
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Document Type: Research Article

Publication date: 2005-01-01

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