INCREASING CAPACITY WITHOUT EXPANDING THE PLANT – OPERATIONS TESTING TO RE-RATE TWO TREATMENT PLANTS

The City of Corona (City) has three wastewater treatment plants (WWTPs) that currently are rated for a total capacity of 15.5 million gallons per day (mgd). To provide capacity for the build-out population, the City took a different approach and considered re-rating two of the WWTPs to maximize existing assets before considering the high capital and operation and maintenance (O&M) cost option of expanding the plants with new facilities. This paper presents the operational testing program (OTP) conducted to verify unit process capacities, lessons learned in conducting operational testing and some of the key recommendations that were made to enable re-rating of the plants with no facility expansion. A similar approach may be used at other plants to maximize the use of existing assets, minimize the need for facility expansions and provide significant cost savings where the following conditions apply:the service area is close to build-outprior to expansion to confirm actual capacities of unit processes, and identify bottlenecksto optimize facilities that may be performing below design and balance performance of primarysedimentation, BOD and nutrient removal and secondary clarification to optimize the treatment trainThe City's Plant No. 1 has two secondary treatment trains – Plant 1A has a step-feed activated-sludge process with primary sedimentation with a rated capacity of 5.5 mgd and Plant 1B has an oxidation ditch process with a rated capacity of 6 mgd. Plant No. 2 has primary sedimentation followed by activated sludge treatment with a rated capacity of 3 mgd. Plant No. 3 is the newest plant, with a 1-mgd membrane bioreactor plant that was designed to expand to 3 mgd in the future.To provide treatment capacity for the build-out population, the proposed rerating of Plants No. 1 and No. 2 would provide an additional 3.5 mgd capacity. The proposed re-rated capacities were 6.5 mgd for Plant 1A, 8 mgd for Plant 1B, and 3.5 mgd for Plant No. 2, providing a combined capacity of 18 mgd. The City required that reliable treatment capacity be provided at the re-rated flows. The term “reliable treatment capacity” is used to refer to the ability to meet required effluent standards with one unit out of service, as required by the California Title 22 recycled water regulations. The aims of the OTP were:Identification and implementation of operational improvements and plant modifications to enhance performance of the City's wastewater treatment plants.Provide documentation and operational support for the proposed re-rated capacities.The re-rating report, scope, and aims of the OTP and the test protocols were discussed with the local regulator, the Santa Ana Regional Water Quality Control Board. The operations testing commenced in March 2003 and was completed in June 2004. The tests conducted were:Primary settling stress tests at Plant 1A and Plant No. 2 — evaluation of primary clarifier performance at higher flow rates, with one basin offline.Plant 1A biological nutrient removal (BNR) test — included testing the aeration basin profile, step feed flow split optimization, capacity testing at higher flow rates and reliability testing with one basin offline.Plant 1B MLSS test — evaluation of oxidation ditch performance at lower mixed-liquor suspended solids (MLSS) concentration and higher flows.Secondary clarifier tests at Plant 1A and Plant 1B — evaluation of secondary clarifier performance at higher flow rates, with one basin offline.A number of key lessons were learned with regard to conducting full-scale operational testing. In particular, it is vital to have the full participation and support of the plant operations and laboratory staff. The testing program needs to be accommodated within the confines of the existing plant setup, and without violating any permit standards, which requires the commitment of the operations staff. The changes that are required and the real-time observable parameters, such as sludge blanket height and turbidity, need to be clear. The sampling requirements to document and evaluate plant performance are also substantial and require close co-ordination with laboratory personnel.The results of the OTP showed that the plants could be re-rated without construction of new facilities, based on implementation of certain recommendations. The recommendations were grouped into operational modifications, short-term recommendations that could be implemented with minimal engineering and design requirements, and longer-term recommendations. Operational modifications included recommendations on MLSS concentrations and return activated sludge rates when secondary clarifiers were taken off line. Short-term recommendations included provision of polymer systems for enhanced primary settling when one sedimentation basin is taken off line. Longer-term recommendations included upgrades to enable operation of the Plant 1A aeration basins in plug flow mode when one basin is taken out of service and upgrades to the aeration system. It was clear that the avoided construction costs for new primary basins, secondary treatment processes and clarifiers equivalent to an additional 3.5 mgd of capacity was significant.