Wet/Cold-Weather Operating Plans

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Communities that have combined collection systems or that are subject to sanitary-sewer overflows during wet weather should consider developing wet-weather operating plans or standard operating procedures (SOPs). A wet-weather operating plan or SOP also benefits
facilities subject to process upset during wet-weather periods. This plan provides operators with a guide to minimize the discharge of pollutants during wet weather and protect their facility from upset.

These plans or SOPs typically focus on determining performance during wet weather as compared to dry weather, determining a facility’s capability to operate at incremental increases in wet-weather flow, and assessing whether unused facilities at the plant can be used to store or
treat wet-weather flows. Also, by keeping accurate records, correlations can be developed between weather events and flows, which is helpful in predicting the impacts of storm events and preparing for expected weather conditions.

Snow melt, rain, and infiltration and inflow (I&I) can drastically affect the way an SBR functions from a microbial standpoint. Influent oxygen levels as high as 5 mg/L, diluted BOD, and cold sewerage temps are all long-term spring occurrences and need to be given serious consideration during the SBR design process. The function of the SBR for nutrient removal requires control over the oxygen level during the various SBR phases. Loss of this control due to long periods of I&I can limit the effectiveness of the nutrient-removal process.

Likewise, a facility may want to develop cold-weather operating plans or SOPs to mitigate treatment impacts during winter months.


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