a. Basic principle: At critically low river flow, BOD and SS loadings are reduced by restricting effluent discharge rates rather than decreasing concentration of pollutants.
b. Must be equipped to retain wastewater during low flow (Q10/7). Use existing ponds or build storage ponds. (Q10/7 = once-in-10-year low flow rate for 7-day period.)
c. Assimilative capacity of receiving stream must be established by studying historical data or estimated using techniques available in the literature (Zirschsky and Thomas, 1987).
periods are controlled by a gauging station in the receiving stream and are allowed to occur during high-flow periods. During low-flow periods, the effluent is stored in the HCR pond. The process design uses conventional facultative or aerated ponds for the basic treatment, followed by the HCR cell for storage and discharge. No treatment allowances are made during design for the residence time in the HCR cell; its sole function is storage. Depending on stream flow conditions, storage needs may range from 30 to 120 d. The design maximum water level in the HCR cell is typically about 8 ft (2.4 m), with a minimum water level of 2 ft (0.6 m). Other physical elements are similar to conventional pond systems. The major advantage of HCR systems is the possibility of utilizing lower discharge standards during high-flow conditions as compared to a system designed for very stringent low-flow requirements and then operated in that mode on a continuous basis. A summary of the design approach used in the United States is shown in Table 5.17. Zirschsky and Thomas (1987) performed an assessment of HCR systems in the United States that demonstrated that the HCR system is an effective, economical, and easily operated system. It was also found to be an effective means of upgrading a lagoon system. Several simple effluent release structures are illustrated in the article.
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