Flow Equalization, Hydrolysis and Nutrient Imbalance A Case Study of a Snack Food Industrial Wastewater Treatment Plant
Abstract:The process wastewater treatment plant (WWTP) of Herr Foods (Nottingham, PA) treats a typical high strength wastewater of the food industry. Herr Foods is a major snack food producer with potato chips and corn chips being the products from which most of their wastewater is generated. The main waste stream to the WWTP contains a lot of potato peels, potato starch and soil from the washing, peeling and slicing processes employed to produce potato chips. Another waste stream coming into the plant contains a high concentration of vegetable oil and caustic generated when the potato and corn chip fryers are cleaned. The major treatment units of the WWTP include a raw wastewater flow equalization tank, Dissolved Air Flotation (DAF) solids removal unit, post-DAF flow equalization tank, plug flow extended aeration bioreactor and final clarifier followed by a lagoon storage system. All of the final effluent is disposed of via spray irrigation.
During the last few years, the plant experienced problems such as pH variation throughout the process units, inconsistent performance of the DAF, low dissolved oxygen in the bioreactor, and sludge bulking. By conducting a comprehensive process evaluation, the causes of these problems were determined, and certain process modifications were proposed to correct the conditions.
The pre-DAF flow equalization tank (raw waste tank) was too large. Because the hydraulic retention time (HRT) was long, the potato starch hydrolyzed and produced a lot of soluble COD including volatile fatty acid (VFA). The VFA reduced the pH, and adversely influenced the performance of the downstream DAF solids removal unit. The COD load to the downstream activated sludge (AS) process was very high. That was because the particulate COD, which should have been removed by the DAF unit, was hydrolyzed in the pre-DAF flow equalization tank to soluble COD, and soluble COD could not be removed by the DAF. On-line monitoring of pre-DAF equalization tank water level showed that half of the volume of the existing tank would provide sufficient flow equalization capacity. Therefore, it was recommended that half of the original pre-DAF flow equalization tank volume be used to reduce the HRT and thus to reducing the solubalization and fermentation occurring there. This could be easily accomplished as the pre-DAF flow equalization tank system was composed of two equally sized tanks capable of being hydraulically separated.
The fryer washing waste stream entered the plant essentially as a slug load purely due to the manner in which the fryers are cleaned. Because it carried a lot of oil and caustic, the COD peak load was very high and significantly increased the pH in the pre-DAF flow equalization tank. The high pH adversely influenced the performance of the DAF unit and the downstream AS process. The tank taken off line which served as half of the pre-DAF flow equalization tank volume was available and was employed to store this oily and caustic fryer wash water waste stream. This waste stream was then slowly dosed into the pre-DAF flow equalization tank over more of an extended period of time. By doing this, the very high peak COD load was equalized. The pH variation in the pre-DAF flow equalization tank was reduced and performance of the DAF was improved.
Although the TOC/TKN ratio was typically adequate from a theoretical nutrient balance perspective, the ammonia concentration in the post-DAF flow equalization tank was low, and a local nitrogen deficiency was detected. Nitrogen source limitation could be one of the reasons for the sludge bulking observed in the final clarifier. The reason of the local nutrient imbalance is that the rate of ammonification (organic nitrogen converted to ammonia) could not match the rate of CBOD and ammonia assimilation to support cell growth. At times when the organic load to the process was ‘normal’ i.e. outside peak snack food production periods, ammonia concentration toward the end of the bioreactor was typically high. Instead of the common practice of adding ammonia, an internal re-circulation was recommended as an inexpensive option to provide the organisms balanced nutrients, reduce the high oxygen requirement at the front end of the bioreactor and reduce the risk of sludge bulking.
The case study showed that biological reactions in the flow equalization, impacts from the caustic wash water stream and nutrient imbalance are the issues that need attention when diagnosing biological process problems in treating wastewater from the food industry.
Document Type: Research Article
Publication date: January 1, 2002
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