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Grenca,plesign Pre-Anoxic Treatment Stage Purpose and Description: This process consists of recirculating a portion of the recirc-blend (or filtrate)from the AdvanTex secondary treatment system to an anoxic zone within the initial primary solids settling/collection chamber or, preferably, in a separate pre-anoxic tank.A pre-anoxic treatment stage tends to balance and lower concentrations.By blending primary treated effluent with AX filtrate, it also provides an environment for denitrifying a portion of the nitrified filtrate. The use of a pre-anoxic stage benefits all applications and is essential for those applications with high-strength waste(organic or nitrogen con- centrations),and restrictive permit limits,as well as applications desiring higher-quality effluent and enhanced overall removal performance. Design Notes and Special Considerations: Orenco recommends the use of a pre-anoxic stage for all projects. For recommendations on sizing of pre-anoxic tankage(typically 1 day QM), see Appendix A. Pre-anoxic tankage volume is a component of the overall primary tankage. For an effluent sewer collection system (i.e.STEP) the pre-anoxic tank is sized at 50%of the values provided in Appendix A for gravity or onsite tankage options. The pre-anoxic return ratio(RNox) is the ratio of flow of the pre-anoxic return loop in relation to the average day design flow. For most applications,the RNox value is equal to 1±and therefore the return flow to the pre-anoxic stage(QRNox) is equal to the Average Day Design Flow(QA). Alkalinity is often added in this stage, because the pre-anoxic return line is a convenient place to add alkalinity while simplifying the overall system layout.The pre-anoxic return line can also be used to introduce supplemental carbon while maintaining a simple design.The estab- lishment of denitrification in this stage reduces organic and nitrogen levels while returning about 50%of the alkalinity consumed during the first stage of secondary treatment(3.57 mg/L alkalinity per 1 mg/L NO3-N denitrified). Consider supplemental carbon addition in the pre-anoxic stage for... • Systems requiring significant total nitrogen reduction (> 85%) • Systems with high nitrogen values in primary treated effluent(Application Types 2,3,&5), resulting in low C:N ratios(<4:1) Orenco offers liquid and dry chemical feed units for adding alkalinity and liquid chemical feed units for supplemental carbon.There are advantages and disadvantages to various alkalinity sources and supplemental carbon products,so consider specific project conditions when making a selec- tion. Flow Equalization Stage Purpose and Description: Flow equalization (EQ)provides stability by leveling out peaks in flow and allowing consistent loading of the treatment system. EQ is strongly rec- ommended for systems with variable flow patterns and restrictive discharge limits. EQ is especially important for systems that have highly variable flow patterns due to usage(e.g., resorts and churches)or collection method (e.g.,conventional gravity collection). The EQ stage consists of a tank or tanks fitted with a timed-dose-controlled pumping system.It follows the primary tank and pre-anoxic tank(if used)and is typically located before pre-aeration/clarification tankage (if used)or a recirculation-blend chamber. Design Notes and Special Considerations: EQ tank sizing recommendations vary for systems with significant fluctuations in flow. For support with EQ tank sizing,contact Orenco. For schools and churches, Orenco typically recommends dividing the system's total weekly flow by six and using this value as the Design Average Flow,with one day allowed for recovery.Using this technique,an EQ tank equal to the Design Maximum Day Flow is generally adequate, but cal- culations should be performed to verify the tank sizing requirement. By their nature,effluent sewer collection systems inherently provide a significant amount of flow equalization.When using this collection method,the addition of EQ tanks at the treatment site is only necessary for systems with extreme flow fluctuations(e.g.,fairgrounds,racing venues,etc.)or highly restrictive permit requirements. Pre-Aeration Treatment Stage Purpose and Description: Pre-aeration reduces organic waste strength prior to secondary treatment,with a typical target reduction of BODS to less than 400 mg/L. NDA-ATX-1 Orenco Systems®•800-348-9843•+1 541-459-4449•www.orenco.com Rev.8,©10/20 Page 12 of 38