Montevallo Wastewater Treatment Plant Improvements

The Montevallo Water & Sewer Board’s existing wastewater treatment facility is nearing the end of its useful life and the flow into the plant is approaching the hydraulic capacity of the treatment units. Due to these factors and the more stringent discharge permit limits being issued by the Alabama Department of Environmental Management for all treatment plants discharging into tributaries of the Cahaba River, the Board retained InSite Engineering to plan, design, and administer the construction of the necessary improvements to the plant.

The required improvements to the plant include a new influent pumping station, a grit removal facility, a dual basin sequencing batch reactor (SBR) treatment process, a post-equalization basin, chemical injection and mixing facilities, filtration, disinfection, blowers, sludge dewatering facilities, an administration building and laboratory, and the conversion of existing facilities into sludge digesters and thickeners. All of the work is being completed under a general construction contract with Morgan Contracting, Inc. from Baker, Florida.

Environmentally Friendly Design

The Board asked InSite to incorporate environmentally friendly and cost effective measures into the design wherever possible. InSite worked very closely with equipment manufacturers to find the most suitable equipment that meets both of these requirements. Some of the selected technologies include:

  • A grit removal system with no moving parts or electrical energy consumption.
  • Solids dewatering facilities which are estimated to reduce the number of loads of sludge hauled from the plant by 81%, and provide an operational cost savings in excess of $500,000 over a 20 year period.
  • BR technology utilizing dissolved oxygen probes and variable speed blowers which are estimated to use 20% less energy than similar systems without these features.
  • An ultraviolet disinfection system which eliminates the requirement for storing and handling of hazardous chlorine gas and sulfur dioxide. The selected system uses low pressure microwave technology, which is estimated to use 90% less energy than similar alternate technologies accomplishing the same level of disinfection.
  • Tertiary filtration which will improve the water quality of the treated plant effluent and make the effluent acceptable for re-use water.
  • A re-use water system which will allow the Board to utilize treated filtered plant effluent for the non-potable water needs at the plant.

Combined, all of these technologies are estimated to generate an annual energy and operational cost savings to the Board of over $250,000 per year when compared to similar alternate technologies achieving the same levels of treatment and disinfection.

The Treatment Process

Wastewater created by residences, schools, churches, hospitals, businesses, industries, and recreational facilities in Montevallo must ultimately be returned to local receiving waters. The new treatment plant will have a capacity to treat 950,000 gallons per day, and a short term surge of up to almost 3,000,000 gallons per day of wastewater.

When wastewater leaves a home or facility, it travels through a series of underground pipes, manholes, and pumping stations until it reaches our wastewater treatment plant. When it arrives at the plant, the wastewater flows into the Influent Pumping Station and is transferred to the Headworks, where a mechanical Drum Screen removes large items such as trash, plastic, rags, sticks, and other similar items. From the Drum Screen, the flow goes through the Grit Removal System and on to the SBR’s.

The SBR has five main operational steps that it goes through to treat the screened wastewater. Step one is the “Mix Fill Cycle”. During this first cycle, raw sewage enters the reactor and mixes completely without the use of aeration. This step is critical at the Montevallo WWTP because of the phosphorous limits imposed on us due to our location in the Cahaba River Watershed. Step two in the process is the “React Fill Cycle”, during which raw sewage continues to enter the reactor, but we add intermittent aeration to help promote the biological conditions needed to begin the treatment process. We have two reactors that operate on a staggered time schedule to allow for maximum operational flexibility. Each reactor is a large concrete tank in which the treatment of the wastewater occurs.

Once the reactor is full, it enters the “React Cycle”. During this phase, we stop filling the reactor, but continue mixing and aeration of the waste. As part of our green infrastructure initiatives, we use dissolved oxygen probes during this phase to measure the oxygen levels in the wastewater. This technology allows us to deliver only the amount of oxygen necessary to maintain the treatment and mixing process without wasting energy. Once the microorganisms have had time to react, we turn off the air and the reactor enters the “Settle Cycle”. During this phase, we turn off the air and mixers and allow the solids to settle to the bottom of the reactor which leaves the cleanest water on top. We adjust the time for the settling cycle based on the process needs and the time required to achieve the most ideal solid/liquid separation.

After enough settling time, the reactor is ready to enter the final “Decant and Sludge Waste” cycle. During this phase, the cleanest water is withdrawn from the top of the reactor by a floating device called a “decanter”. Once the water is withdrawn, we spend a few minutes pumping the heaviest solids, known as “sludge”, from the bottom of the reactor. Some of this sludge is later returned to the reactor to ensure the integrity of our process, and some of it is transferred into the Sludge Digester.

Our new Aerobic Sludge Digester is actually our old wastewater treatment plant Aeration Basin. By making minor modifications to the old basin, we were able to significantly increase our digestion volume for minimal capital cost. Aerobic digestion is a bacterial process that occurs in the presence of oxygen. Under aerobic conditions, our bacteria rapidly consume organic matter and convert it into carbon dioxide. Once there is a lack of organic matter, the bacteria will die and be used as food by other bacteria, which reduces the amount of solids we have to dispose of.

Once the sludge has gone through the Digester, we will process it through the centrifuge in our new Sludge Handling Building. This centrifuge will remove the excess water from the sludge, and significantly reduce the volume of solids that we have to haul off of the site and dispose of. We conservatively estimate that this step in the process alone will save us in excess of $500,000 in the next 20 years.

Once the cleanest water is decanted from the SBR reactors, it is purified even further by flowing through a new filtration system called a disc filter. These types of filters use a cloth media to provide discharge water of high enough quality to be re-used for other purposes. In fact, we will be using some of this water to irrigate the site and wash down equipment and facilities around the wastewater treatment plant.

The final step in the treatment process, after filtration, is disinfection using ultraviolet (UV) light. The technology at our plant uses microwaves to energize low-pressure high-ouput lamps that are estimated to use up to 90% less energy than other similar technologies. Once disinfection is complete, the treated filtered disinfected water is discharged back in to the receiving stream and returned to the environment.