DESIGNING WWTPS TO Meet All Discharge Requirements.
Since these projects were funded by Clean Water Grants, the state and federal agencies required that the facilities be designed to meet all waste discharge requirements at all times. We gave special consideration to the Bay Area's known high peak wet-weather flow conditions and used numerous unusual approaches to design, build, and operate the wastewater facilities that were part of the Marin-Sonoma County regional program and other Bay Area facilities. While the projects focused on providing for high wet-weather flow conditions, they also had to be cost-effective.
In February 1998, the El Nino weather pattern brought record rainfall to California and provided a meaningful response to the question, "will the project meet all the waste discharge requirements at all times?" According to the National Weather Service, February 1998 rainfall set new records at over a dozen locations in California. In parts of the Bay Area, wastewater flows for the month ran as high as 400 percent of average.
CENTRAL MARIN SANITATION AGENCY
One of the most significant tests of WWTP performance during the peak of the February storms occurred at the Central Marin Sanitation Agency (CMSA) plant. Completed in 1985, CMSA's entirely new wastewater treatment facility cost $45 million and was designed to treat wastewater that met all state and EPA requirements. The following features were designed to accommodate exceptionally high peak wet-weather flow conditions:
* Plant designed for average dry-weather flows of 10 mgd, with accommodation for peak wet-weather flow rates of up to 125 mgd.
* Headworks and grit chambers designed to accommodate 125-mgd flow rates.
* Primary sedimentation tanks designed to accommodate 125-mgd flow rates, but with high overflow rates (2,500 gpd/sq ft) and short detention time (less than one hr).
* Facilities to provide ferric chloride/polymer feed during flow rates greater than 40 mgd to assist in solids coagulation ahead of primary sedimentation tanks.
* Secondary, dual biologic treatment units, fixed film reactors/activated sludge, secondary clarifiers, designed to accept up to 30 mgd.
* Overflow weirs and channel from primary sedimentation tank outlets to convey flows above 30-mgd flow rate around secondary biologic units directly to chlorine contact tanks.
* Chlorine contact tanks and chlorine feed facilities designed to accommodate 125 mgd.
* Solids handling facilities, digesters, thickeners, and centrifuges designed on the basis of 10-mgd average dry-weather flows.
The plant's flexible, innovative design, coupled with skillful operation by plant staff under the direction of Phil Fry, CMSA superintendent, allowed CMSA to handle high flows during the fullest force of the El Nino rains and to more than meet treatment requirements. During February, at the height of the El Nino rains, the average daily wastewater flow was nearly four times the normal, while the highest flows were nearly 10 times the average. During the peak five days, daily flows for the plant averaged more than six times the normal, but CMSA more than met treatment requirements with average effluent BOD=15 mg/Land SS=25 mg/L, and removals of 92 percent BOD and 89 percent SS.
PALO ALTO'S RWQCP
The flows at Palo Alto's Regional Water Quality Control Plant (RWQCP) did not reach the extreme peak-to-average flows that were experienced by CMSA during the highest wastewater flows in February 1998. Nevertheless, the RWQCP also demonstrated the effectiveness of strategic design and operation to meet sustained, exceptionally high flows while meeting more stringent requirements.
The difficulties associated with high wastewater flows at the Palo Alto RWQCP are compounded by additional restrictions that require tertiary treatment at all times. Not only is the RWQCP subject to the 85 percent BOD/SS removal requirement, but BOD and SS residuals in plant effluent are also limited to a monthly average of 10 mg/L. In addition, the plant effluent must be nitrified and stay within a monthly average residual ammonia limit of 3 mg/L and turbidity must be less than 10 ntu.
The major process units at the Palo Alto treatment facilities, primary sedimentation tanks, fixed film reactors, aeration basins, and multimedia filters were designed for average dry-weather flows of 30 mgd. In addition, the regional sewerage system has a unique built-in feature that can accommodate high flows. The transport sewer that conveys wastewater from the cities of Mountain View and Los Altos to the Palo Alto plant was designed with excess flow capacity. That extra capacity can be used strategically to store and buffer some of the high flow through the influent pumping plant before it goes to the treatment plant. The plant was skillfully operated by plant staff under Superintendent Bill Miks, whose staff also developed a program that partially bypasses process units, particularly in operating the filters. In February 1998, the combined use of these two strategies gave plant personnel excellent control and produced results that were beyond expectations.
Peak flow for the month was 260 percent of normal, and average flow during the five wettest days was nearly twice the design capacity. Nevertheless, plant personnel were able to maintain a continuous level of tertiary treatment throughout the entire month, including essentially complete nitrification. The plant also more than met and maintained tertiary requirements for the entire month.
OTHER INNOVATIVE PROGRAMS
Innovative and economical features were also incorporated into other Bay Area wastewater treatment plants that were designed and constructed as part of the Marin-Sonoma regional program. For instance, the Novato Sanitary District in northern Marin County had the same need to account for exceptionally high peak wet-weather flows vs. average dry-weather flow (9:1). The Novato District's needs were addressed through the use of final filters, with high hydraulic loadings, up to 15 gpm/sq ft.
In designing improvements for both of the Novato District's plants (Novato and Ignacio), Kennedy/ Jenks anticipated that (at a certain maximum flow rate), a portion of primary effluent would be bypassed around the secondary biologic units and go directly to filters that could operate at high hydraulic loadings. This approach was based upon pilot plant studies that demonstrated that during high flows such as occur during winter months, unexpected amounts of filterable solids caused high chemical BOD. By filtering out the solids, the amount of residual BOD could be reduced. The filtered effluent could then be combined with treated effluent from the secondary biologic oxidation elements of the plant to meet water quality requirements. At the main Novato District plant, provision was made for equalization of incoming flows by building an expanded basin in an old sludge holding lagoon that was no longer in use.
In southern Mann County, the Sausalito Sanitary District's wastewater treatment plant improvements project included two over-sized secondary sedimentation tanks and the installation of upflow sand filters. Both of these elements were designed with special emphasis on the needs associated with high peak flow conditions that were up to eight times the average flow (8:1).
Mr. Jenks is the Senior Consultant at Kennedy/Jenks Consultants, San Francisco, California. He has over 50 years of experience in wastewater engineering and was instrumental in designing the wastewater treatment plants mentioned in this article.
MONITORING RESULTS FROM CMSA FOR FEBRUARY 1998 Conditions During February 1998 Design avg. dry-weather flow 10 mgd Actual avg. daily flow for month 38 mgd Actual peak day flow rate 96 mgd Actual avg. daily peak flow rate 51 mgd Actual avg. daily flow during peak five days 61 mgd Results of Operation Avg. BOD for month 9 mg/L Avg. effluent SS for month 19 mgd Avg. effluent BOD during peak five days 15 mg/L Avg. effluent SS during peak five days 25 mgd Avg. removal BOD for month 92% Avg. removal SS for month 89% MONITORING RESULTS FROM PALO ALTO FOR FEBRUARY 1998 Conditions During February 1998 Design avg. dry-weather flow 30 mgd Actual avg. daily flow for month 44 mgd Actual peak day flow rate 80 mgd Actual avg. daily peak flow rate 55 mgd Actual avg. daily flow during peak five days 56 mgd Results of Oper3ation Avg. effluent BOD for month 2.9 mg/L Avg. effluent SS for month 3.6 mg/L Avg. effluent BOD during peak five days, 3.7 mg/L Avg. effluent SS during peak five days 5.5 mg/L Avg. effluent ammonia for month 0.2 mg/L Avg. effluent turbidity for month 1.2 ntu Avg. BOD removal for month 98% Avg. SS removal for month 98%
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|Author:||Jenks, John H.|
|Article Type:||Statistical Data Included|
|Date:||Dec 1, 1999|
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