Kansas plant clarifies new overflow solution.
During wet weather periods in Lawrence, Kan., up to 40 million gallons per day (mgd) of excess wastewater flow are now being treated with the nation's first full-scale ballasted flocculation facility for sanitary sewer overfow (SSO). Completed this past summer, the plant expansion included improvements to address future growth, new regulatory requirements, and facility rehabilitation needs for the design year 2020. Toward that end, the $9 million project provided an excess flow treatment facility for projected peak flows during storm events.
Originally, the plant's main liquid treatment process was designed to treat 25 mgd. However, the projected 2020 peak flow is expected to reach 65 mgd during wet weather events (based on a 10-year storm event). Consequently, city leaders knew they had to find a way to treat 40 mgd of excess storm flow. To solve this problem, the city retained Black & Veatch Corp., Kansas City, Mo., to evaluate options and to design and manage construction of a new state-of-the-art addition to its facility.
Because Lawrence has separate sanitary and stormwater sewer systems, the city must comply with current and anticipated state SSO regulations for the excess wastewater flow. After studying options to handle the excess flow--including new high-rate clarification technologies and storage options--in 1999 Black & Veatch recommended the application of ballasted flocculation to achieve high-rate clarification as the best long-term solution.
Subsequently, the plant upgrade and expansion also included the addition of a flow splitter/screening facility, disinfection basin, and chemical-feed facility. Fine screens now protect the new excess flow treatment process equipment, removing trash, debris, and rags from the process stream.
THE PEAK FLOW CHALLENGE
Since heavy rainstorms can produce peak wastewater flows 5 to 10 times greater than flows produced during dry-weather conditions, die Kansas Department of Health and Environment requires that all SSO discharges into the Kansas River must meet National Pollutant Discharge Elimination System (NPDES) permit limits. But the city could not obtain a water quality variance for discharges during storm events because peak flow discharges can occur when the flow in the Kansas River is low. Since the river has a large tributary drainage area, isolated storm events in Lawrence can still cause peak flow discharges even when the water level is otherwise low.
In Lawrence, the site also posed unique challenges, being adjacent to the floodway and within the Kansas River floodplain. On the north side of the plant, for instance, the floodway precluded expansion while elsewhere in the floodplain rigorous permitting approvals required by federal, state, and local governments and water resources agencies amounted to significant hurdles, as well. One option initially considered for handling excess flow was the construction of SSO storage basins with a combined capacity of more than 30 mgd. Potential odor problems from such basins were a concern because the plant is near both residential and commercial areas. Removal of solids from the storage basins after rain events was also a potential problem.
On the positive side, flows in excess of the 25 mgd handled by the plant's biological treatment process are diluted by storm events and therefore have lower concentrations of total suspended solids (TSS) and biochemical oxygen demand (BOD). The lower pollutant concentrations inherent in storm flows provide for the potential use of high-rate clarification technologies to achieve permit limits.
A HIGH-TECH SOLUTION
Based on a life-cycle cost evaluation, ballasted flocculation was selected to provide the additional capacity required for peak storm flow treatment in Lawrence. This technology, initially used in Europe for potable water treatment, has recently been introduced to the U.S. market for treatment of SSOs and combined sewer overflows.
Ballasted flocculation is a high-rate physical-chemical process for enhanced reduction of TSS and BOD. It is similar to conventional coagulation, flocculation, and sedimentation technology used in water treatment plants because a coagulant is used for destabilization of suspended materials entering the process and a polymer flocculent is added to aggregate the solids into larger masses. Then, the resulting floc is removed via settling.
In the ballasted flocculation process, however, microsand--fine sand similar to silica powder--is added as seed for the development of high-density floc. Ballasted by the relatively high-density microsand, the floc settles more rapidly, allowing for the removal of solids at a very high surface overflow rate. In addition, the process can be rapidly initiated and optimized despite variations in flow and water quality.
Ballasted flocculation permits wastewater treatment, without the need for storage, at a very high rate: tip to 60 gallons per minute per square foot of basin surface area. This is as much as 40 to 60 times higher than conventional clarifier loading rates. As a result, the technology requires 1/10 of the space required for conventional clarification and 1/15 of that required for storage basin alternatives.
SMALL SIZE, RAPID RESPONSE
Lawrence's new ballasted flocculation treatment facility provides permit compliance in a small footprint. It consists of two 20-mgd treatment trains constructed within an 85 x 45 foot area. Each treatment train includes a coagulation chamber, an injection chamber, a flocculation (maturation) basin, and a settling zone.
Ferric chloride is added as a coagulant to the wastewater flow at the excess flow splitter/screening facility ahead of the excess flow treatment basin. A diffuser disperses the ferric chloride at the entrance of each excess flow treatment basin influent pipe ahead of the coagulation chamber. There, the coagulated water enters tire injection tank, where it is mixed with polymer and microsand. Most of the floc forms in the flocculation (maturation) tank, then the ballasted floc settles out in the sedimentation basin, which is equipped with lamella plate settlers.
The clarified water flows over the effluent weirs and discharges to the excess flow chlorine contact basin, where the flow is disinfected by sodium hypochlorite and dechlorinated with sodium bisulfite. Fully treated excess flow effluent is then blended with the plant's main effluent and conveyed by gravity through an outfall pipeline for discharge into the Kansas River.
Meanwhile, the sand-sludge mixture collected at the bottom of the settling tank is pumped to hydrocyclones. The separated microsand is concentrated and discharged through the bottom of the hydrocyclones. Most of the microsand is recycled into tire excess flow treatment basins for further flocculation. The sludge separated by the hydrocyclones flows by gravity back to the head of the plant to the influent pumping station to be treated by the main biological treatment process.
Today, treatment of excess flows during storm events is fully automated in Lawrence. A new computer supervisory control and data acquisition system minimizes operator attention requirements and rapidly responds to a full range of flow and water quality scenarios with real-time data as the plant receives storm flows. The excess flow treatment system can be started up and optimized within 15 minutes--or more quickly if necessary with the un-aged polymer mode. The system also monitors collection system sewers and pumping stations to allow city staff to predict the onset of high-flow conditions based on real-time data.
FACILITY PASSES FIRST TESTS
In June, the facility conducted a performance test for the constructed full-scale excess flow treatment facility. Each treatment train was tested independently at full capacity on successive days. Primary basin effluent was used with 50% of the primary basin capacity turned off to simulate storm event wastewater flow conditions. In addition, storage of some of the primary basin effluent was neccessary to obtain the large volume of flow required to conduct the test.
The test yielded excellent results. Whereas the city's NPDES permit requires a maximum effluent TSS of 45 mg/liter for the ballasted flocculation system prior to blending with the main effluent, testing revealed an average effluent TSS range of just 20 to 30 mg/liter and TSS removal rates of 65% to 80%. The city expects even better results when processing the more diluted wastewater produced during actual storm events.
Looking toward the future, the new ballasted flocculation plant was designed with an eye on inevitable expansions. The facility layout, buildings, and process piping were laid out to minimize complications, site constraints, and costs for future growth. Already, peak flow treatment needs for today through the year 2020 have been met, with flexibility built in to accommodate continued population increases.
Construction costs for the ballasted flocculation treatment basins and equipment, including buildings for the sludge primping station and microsand storage, totaled approximately $4.4 million, or $0.11 per gallon of capacity. Total construction costs for all of the excess flow facilities were approximately $9 million, or $0.22 per gallon of capacity. With the fine screen building sized tot addition of a future screen, the treatment facility designed for flexible growth, and the probability of lower costs for this new technology likely down the road, the cost per gallon to expand these facilities in the future should be even more reasonable.
--Wagner is the assistant utilities director in Lawrence, Kan. Schultze is the project manager and Keller is project engineer with Black & Veatch Corp., Kansas City, Mo.