Real Time Control (RTC) operation strategies has emerged as one of the key technologies to maximize inline storage and avoid sanitary and combined sewerage overflows (SSO and CSO). Availability of large inline storage facilities allows maximum benefit from RTC operation. Olentangy Scioto Interceptor Sewer (OSIS), the main combined sewer flowing through downtown Columbus, overflows into the City’ Olentangy and Scioto Rivers during large storm events. The City of Columbus has built the OSIS Augmentation Relief Sewer (OARS), 23,300 ft of 20’ diameter tunnel, with four shafts, treatment plant gravity connection, and a dewatering pumping system. Under the designed level of service of OARS, discharge from 10 CSOs along OSIS will be mitigated to up to a 10-yr flow event, and nearly two billion gallons of sewage will be kept from overflowing into the Olentangy and Scioto River each year.
There are two potential operation strategies for the OARS tunnel; storage and conveyance, or storage only and dewatering after the wet weather flow event passes and after Jackson Pike or Southerly WWTPs has treatment capacity. Through storm analysis and model experiments, this presentation will answer the following question by linking real-time rainfall data with OARS dewatering strategy: Is there a storm threshold above which OARS is expecting to overflow? How to manage OARS operation to avoid overflowing at large storms? How to identify characteristics of back-to-back storms that could impact the dewatering operation, CSOs activation and treatment plants bypass? What are the pros and cons for different OARS dewatering strategy?
Through statistical analysis, the historical storm events and OARS inflows that cause OARS to fill were categorized into focus groups. A 20 years of historical storm data simulation showed that only 123 of the 801 events that had OARS inflow were expected to completely fill the OARS storage (>63.8 MG). It was observed that among all the storm events with tunnel inflow less than 63.8MG, storm volume over Columbus combined sewage area was below 1.8 inches in 24 hours. Accordingly, up to 85% of the inflow events into OARS storage and dewatering after the storms is adequate operation and ensures no potential bypass at the treatment plants. The storage operation mode relaxes the operators’ decisions by avoiding complex operation protocol during storms and avoids stressing the WWTP facilities. The 123 events that were expected to fill OARS were further analyzed to propose an OARS storage/conveyance operation strategy when rainfall is more than 1.8 inches in 24-hr. The storage/conveyance operation rules used careful coordination between available real-time data including available treatment capacity, forecasted rainfall, and tunnel remaining capacity. The operation allows OARS to dewater early when future storm events are forecasted, which reserves inline storage for upcoming wet weather flow and further reduces CSOs. Results showing impact on CSOs level of service and bypasses at the treatment plants supports the preferred storage and conveyance as needed strategy over the original storage operation.