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DESIGNING WWTPS TO Meet All Discharge Requirements.

"But will these wastewater projects meet all the Waste discharge requirements and do so at all times?" This was the question asked by the technical advisory committee of a regional wastewater program developed for east Marin and south Sonoma counties in California. The plants were upgraded in the late 1980s when many new plants or plant expansions were being built to meet new waste discharge requirements under the Clean Water Act. The same question was asked of other wastewater projects undertaken in the San Francisco Bay Area at that time. Kennedy/Jenks Consultants designed several Bay Area plant upgrades during this period, including the Central Marin Sanitation Agency, the Palo Alto Regional Water Quality Control Plant, the Novato Sanitary District, and the Sausalito Sanitary District. Once constructed, these plants performed well and had an excellent history of meeting waste discharge requirements. But it was not until early 1998 that their ability to handle extreme wastewater flows was actually tested. Des igning wastewater treatment facilities to meet all waste discharge requirements is particularly challenging for agencies in the San Francisco Bay Area, where many sewer agencies have unusually high peak wet-weather wastewater flow conditions during winter months. These high flows are caused by old sewers with cement joints, combined with high groundwater, and direct entry problems. Literally meeting all waste discharge requirements in a facility where peak wet-weather flows can be up to ten times higher than the average design flows was a challenge. At the same time, the EPA requires that at least 85 percent of SS and BOD be removed, and that the monthly average residual SS and BOD remain below 30 mg/L. Meeting such restrictive waste discharge requirements during periods of sustained, extremely high wastewater flows is difficult both from an engineering and economic standpoint.

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.
Publication:Public Works
Article Type:Statistical Data Included
Geographic Code:1USA
Date:Dec 1, 1999
Words:1683
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