ORP and pH Based Control of SBR Cycles for Nutrient Removal from Wastewater

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ABSTRACT
Continuous monitoring of multiple process functions were conducted involving removals of BOD, TSS and nutrients, in each cycle of the SBR reactor at the wastewater treatment plant of the City of Carrabelle, FL. On line, real time, monitoring of pH, oxidation reduction potential (ORP), dissolved oxygen (DO), ammonia-N, nitrite-N, nitrate-N and orthophosphate-P demonstrated excellent correspondence between these parameters and the individual biochemical reactions in each step. Subsequently, the SBR cycle control logic was changed on a trial basis from time and DO based, to ORP based. This led to a superior performance of the SBR reactors and has the potential of resulting in a better energy efficiency while maintaining the effluent quality complying with the State of Florida regulation for BOD, TSS, TN, TP of 5, 5, 3 and 1 mg/L, respectively.

INTRODUCTION
The City of Carrabelle is located in the Florida panhandle on the Gulf of Mexico. It used to be served by a 0.30 MGD extended aeration type activated sludge wastewater treatment plant. The city decided to upgrade their treatment facility to handle the projected growth of population and the stringent nutrient control regulation implemented by the State of Florida and Franklin County. Baskerville Donovan, Inc. (BDI) designed and managed the overall design and construction of the new facility. Siemens Water Technologies designed the control system for the SBR and supplied the internal parts of the two circular, concrete reactors. The new 1.2 MGD treatment system started up in October 2005.

The design influent conditions are – flow = 1.20 MGD (peak = 3.6 MGD), BOD = 255 mg/L, TSS = 225 mg/L, ammonia-N = 23 mg/L, TKN = 35 mg/L and total phosphorus = 6.3 mg/L; and the discharge permit requires the effluent BOD, TSS, TN and TP of 5, 5, 3 and 1 mg/L, respectively on a monthly average basis. The wastewater collected in a network of sewer lines enters the plant through a headwork structure. The influent, free from grit and screenings, then enters two parallel SBR reactors of 1 MG each, designed to operate in a phase difference of 180o. The decant streams from the two SBRs collect in a downstream equalization tank, which feeds a disc filter unit to remove particulate matter carried over by the SBR decant streams in order to fulfill the effluent requirements. Before its final discharge, the treated effluent is disinfected by application of sodium hypochlorite solution in a chlorine contact structure. The discharge has the option of being applied in an onsite spray field, sold to a nearby correctional facility for
laundry and toilet flushing or disposed at a local golf course. However, coliform examination is essential before correctional facility or golf course application. In case of excessive flow due to wet weather there is a provision to divert a part of the influent into a large equalization tank on site.

The plant monitors daily influent and effluent BOD and TSS; and effluent total nitrogen (TN), nitrate-N, and total phosphorus (TP) from composite sampling and reports their weekly average values. To aid in process control, the disinfected final effluent quality at the Carrabelle WWTP is continuously monitored by an on-line Chem Scan Process Analyzer marketed by Applied Spectrometry Associates, Inc. of Waukesha, WI. This instrument continuously measures and displays the effluent ammonia-N, nitrite-N, nitrate-N and orthophosphate-P in the effluent.

The wastewater treatment unit operations and processes in an SBR are separated by time. Each cycle treats a new batch of influent in multiple, sequential steps. A fresh batch of influent is introduced into the SBR tank in the fill step. It mixes up with the treated liquid remaining in the tank from the previous cycle, such that the feed constitutes a part of a given batch. Primarily the fill step is anoxic, during which the nitrate formed in the earlier cycle and remaining in SBR tank after decant step, is removed by denitrification. The next part of the feed step is an anaerobic react step, during which fermentation of the organic matter in the raw sewage takes place in the absence of air. Simultaneously, the polymeric phosphorus bound with the bacterial cells release as inorganic orthophosphate ions. This is followed by aerobic react step. During this step all the BOD removal and nitrification takes place by aeration. Phosphorus is taken up by the biomass at an enhanced rate. At the conclusion of the react step, the aeration and mixing of the liquid is stopped and sludge is allowed to settle under quiescent condition. After settling, the clear liquid on the top of the sludge blanket is decanted out as treated effluent. This completes one full cycle of an SBR. These cycles are continuously repeated in sequence. The ability to separate the individual nutrient control reactions into individual steps in a given cycle is one of the distinct advantages of an SBR over its continuous flow counterpart.

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