November 2010, Vol. 22, No.11

Problem Solvers

Still finding gold out west

Dynamic approach saves money on capital improvements

Problem: Master plan recommends expensive additions and upgrades to increase capacity.
Solution: Hydraulic model with real-time controls identifies solutions to eliminate need for capital improvements.

 

The bustling city of Santa Rosa, Calif., situated approximately 89 km (55 mi) north of San Francisco, recently began preparing for major capital improvement upgrades to its sewer system to address growth and wet weather capacity issues.

In 2005, the city completed a sanitary sewer master plan that called for an $80 million parallel trunk sewer pipe to help prevent problems expected from meeting maximum flows during wet weather. The plan used a steady-state hydraulic model and focused on the collection system. It did not take into account the effect the treatment plant’s capacity limitations had on the collection system or the effect recommended treatment system expansions would have on the collection system, said David Guhin, deputy director of engineering at the City of Santa Rosa.

By the beginning of 2008, the city’s population was estimated to be approximately 160,000. At the time, the city was operating and maintaining a sanitary sewer system spanning approximately 950 km (590 mi) over a 104-km2 (40-mi2) service area, a 52,990-m3 (14 million-gal) remotely controlled wet weather storage facility at the West College Storage Facility (WCSF), and the 212,000-m3/d (56-mgd) Laguna Wastewater Treatment Plant (LTP), with an additional 49,200 m3 (13 million gal) of equalization storage at the treatment plant site.

During heavy rain, flows exceeded the capacity of LTP and the 1650-mm (66-in.) Llano trunk sewer pipe backed up. Historically, the plant was able to use the WCSF to reduce flows to a manageable level both at the plant and in the trunk, which also reduced the risk of sanitary sewer overflows (SSOs). But as the city increases capacity at the plant to accept more flow, the burden will shift to the trunk, potentially resulting in backups and SSOs.

WCSF minimizes surcharging in the trunk line and controls flows to LTP by managing flow from an approximately 4000-ha (10,000-ac) sewer basin. LTP’s equalization basins allow the operational flexibility of additional storage if peak flows exceed the plant’s treatment capacity. Once the equalization basins reach 20% of storage capacity, the WCSF gate closes, diverting flow into a 16,000-m3 (4.2 million-gal) concrete storage basin that is interconnected with an additional 38,000 m3 (10 million gal) of storage in an adjacent dirt basin.

 

Model building

The city decided to re-evaluate its existing facilities, looking at the treatment, storage, and trunk together to optimize performance and to potentially delay, reduce, or eliminate the large capital improvement recommended in the master plan. To do this, the city worked with its consultant, Malcolm Pirnie Inc. (White Plains, N.Y.) to construct and calibrate a skeletal dynamic hydraulic model using the MWH Soft (Broomfield, Colo.) INFOSWMM system. The model incorporated real-time control modeling calibrated to the area’s winter 2007 to 2008 conditions.

Before testing potential solutions, the city used the model to evaluate current performance for 1-, 2-, 5-, 10-, and 25-year storms for both existing and future sanitary flows. The model showed that both the 25-year storm event with existing sanitary flows and the 10- and 25-year storms with sanitary flows expected by the year 2030 caused significant surcharging and overflows along the trunk line. These specific storm events were then used to evaluate the performance of each proposed alternative.

 

Alternative testing

The city then calibrated the model to evaluate two alternatives for managing peak wet weather flows and mitigating surcharge conditions, one incorporating the parallel trunk line proposed by the 2005 master plan and another using expanded storage. The model simulated the interconnected operation of WCSF and LTP; the model’s real-time controls simulated the approximate filling and draining of WCSF and the storage treatment operation of LTP.

For the model to evaluate the 11-km (7-mi) parallel trunk line that would extend from just downstream of WCSF to LTP, the model developers assumed the same invert and slope as the existing trunk line. In this model, to balance flow between the parallel lines, cross-connected pipes were set up every 1.6 km (1 mi) — this added up to 10 152-m (500-ft) pipe segments.

For the second alternative, which involved adding storage to WCSF and optimizing its operation, flows from a permanently mounted flowmeter just downstream of WCSF were used as the trigger to open and close the gate and drain the facility instead of the current influent flows to LTP. By moving this decision point 11 km (7 mi) closer to WCSF, the storage facility can respond approximately 3 hours sooner to flow variations entering the Llano trunk line, improving efficiency, reducing peak flows, and using less WCSF volume. This change enabled the storage facility to provide the same amount of benefit with less storage volume, Guhin said. The wet weather conditions were then run in the model to determine the size of the additional storage volume WCSF would need to avoid surcharging and overflows on the existing Llano trunk.

 

Decision-making

After running both alternatives, the model showed that the parallel trunk line could accommodate flows only up to the 5-year storm without surcharging the existing trunk; but adding storage capacity could accommodate flows up to the 10-year storm and projected 2030 sanitary flows. The model analysis revealed that the overflows issues arise from wastewater bottlenecking at LTP. This suggests that planned upgrades at the LTP could be delayed as they were only needed to accommodate extreme wet weather at projected 2030 flows.

The master plan recommended increasing LTP treatment capacity to 337,000 m3/d (89 mgd). When incorporating this capacity with both alternatives, surcharging and overflows along the entire Llano trunk line were eliminated for even the most extreme scenarios.

Since both the construction and storage alternatives were able to remove surcharging and overflows for 25-year storm conditions when LTP treatment capacity was expanded, cost became the deciding factor. The city chose to expand storage because it was significantly less expensive, potentially saving $50 million in capital costs and eliminating environmental impacts from constructing a parallel trunk line.

“This approach was a creative solution to the problem of dealing with the impacts to our collection system and treatment plant during wet weather events while at the same time minimizing the capital investment,” Guhin said.

 

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