Town of Exmore Wastewater Treatment System

Basics Sewage Treatment

Town of Exmore Wastewater Treatment Plant

The Town of Exmore has constructed a Wastewater Treatment Plant on the east side of the Town’s corporate limits. The Wastewater Treatment Plant is located on the west side of Seaside Road approximately ¼ mile north of the intersection of Willis Wharf Road and Seaside Road. The Treatment Plant occupies approximately 30 acres.

The treatment plant receives wastewater from the Virginia Street community and the business located in the Town’s corporate limits. The wastewater treatment system consists of the following:

1.                  120 septic tank effluent pump systems

2.                   Fluidyne ISMA^TM Sequencing Batch Reactor

3.                   Geoflow wasteflow® drip irrigation system

The wastewater treatment plant has a designed hydraulic capacity of 60,000 gallons per day (GPD). The treatment plant is currently operating at approximately 20% of its capacity.

The treatment plant is currently operating under a Conditional Operating Permit issued by the Virginia Health Department (Permit number 02-100-0070). A final Operating Permit will be issued after the conditions of operation have been met by the Town of Exmore.

Why is Sewage Treatment Important?

Effective sewage treatment prevents a variety of ailments that can be spread by exposure to pathogens that can be present in untreated sewages, and thus helps prevent disease. Discharges of untreated sewage can contaminate groundwaters and surface waters used for drinking, recreation, and fish and shellfish fisheries.

 Untreated sewage from failed conventional septic systems or sewage discharged directly into the environment can percolate into groundwater, contaminating drinking-water wells with pathogens. The discharge of untreated sewage to streams can spread disease through direct contact, making such streams unfit for forms of recreation that involve skin contact with the water such as swimming and boating. Discharged, untreated sewage also can damage the receiving streams' ability to support healthy, living communities of aquatic organisms and can contaminate fisheries.

General Principles of Sewage Treatment

Raw sewage and septic wastewaters contain a variety of contaminants.

Most sewage treatment technologies operate by combining basic physical, chemical, and biological processes.

Primary Treatment (Anaerobic Selector Chamber)

Primary treatment (Influent chamber) removes solid chunks and particles from raw sewage through gravity separation. The partially-treated liquid discharged from primary treatment is called primary effluent. The influent chamber utilizes anaerobic digestion as the first treatment process at the Exmore Wastewater Treatment Plant

Anaerobic Digestion of Sewage

The treatment of sewage is largely a biochemical operation, where chemical transformations of the sewage are carried out by living microorganisms. Different environments favor the growth of different populations of microorganisms and this in turn affects the efficiency, end products, and completeness of treatment of the sewage. Sewage treatment systems, whether they are standard septic systems or more advanced treatment technologies, attempt to create specific biochemical environments to control the sewage treatment process is based on the type of biochemical transformation that occurs.

Three basic types of biochemical transformations occur as sewage is treated.

1^{ST -}     The removal of soluble organic matter. This is composed of dissolved carbon compounds such as detergents, greases, and body wastes, which make up much of the BOD content of the sewage.

2^ND-   The digestion and stabilization of insoluble organic matter. These are the sewage solids, such as body wastes and food particles, which make up the remainder of the BOD.

3^RD -   The transformation of soluble inorganic matter such as nitrogen and phosphorus.

The two major biochemical environments in which sewage treatment is carried out are termed anaerobic and aerobic environments.

anaerobic environment is one in which dissolved oxygen is either not present or its concentration is low enough to limit aerobic metabolism.

aerobic environment is one in which dissolved oxygen is available in sufficient quantity that the growth and respiration of microorganisms is not limited by lack of oxygen.

The biochemical environment has a profound effect upon the ecology of the microbial population which treats the sewage. Anaerobic conditions favor the growth of primarily bacterial populations and produce a different variety of end products; Aerobic conditions tend to support entire food chains from bacteria up to rotifers and protozoas. These microbes breakdown organic matter using many metabolic pathways based on aerobic respiration with carbon dioxide as the main end product.

Solids in sewage contain large amounts of readily available organic material that would produce a rapid growth of microorganisms if treated aerobically. Anaerobic decomposition is able to degrade this organic material while producing much less (approximately one-tenth) biomass than an aerobic treatment process. The principal function of anaerobic digestion is to stabilize insoluble organic matter and to convert as much of these solids as possible to end products such as liquids and gases (including methane) while producing as little residual biomass as possible. It is for this reason that sewage treatment in a conventional septic tank is designed to be an anaerobic process. Organic matter treated anaerobically is not broken down to carbon dioxide; final end products are low molecular weight acids and alcohols. These may be further converted anaerobically to methane or, if sent to an environment (such as the leaching field) where aerobic bacteria are present, further broken down to carbon dioxide. Anaerobic digestion of organic matter is also a much slower process than aerobic digestion of organics and where rapid digestion of organic matter is needed an aerobic treatment process must be used.

As discussed above, an anaerobic environment is also necessary for denitrification, as the bacteria which carry out this process require anaerobic conditions to reduce nitrate to nitrogen gas. Many nitrogen-removal technologies are designed to provide an anaerobic treatment chamber as part of the treatment process.

Aerobic Treatment of Sewage

As the name implies, this process utilizes aerobic bacteria to break down sewage. The principal advantage of aerobic sewage treatment is its ability to rapidly and completely digest sewage, reducing BOD to low levels. This process is used primarily to reduce BOD and, in systems that remove nitrogen, to nitrify the waste so that it can later be denitrified. Because the BOD in raw sewage is usually high, and available oxygen is rapidly consumed by the sewage, most aerobic treatment units are designed to supply supplemental oxygen to the sewage to keep the treatment process aerobic. The activated sludge process is similar to suspended culture in that it also utilizes the resident population of bacteria in the solids and sludge in the treatment unit, again, usually by mixing of the sewage so that the bacteria are kept in suspension. In the activated sludge process, however, there are usually periods where mixing ceases and the solids are allowed to settle. It is then assumed that the sludge will become anaerobic and the anaerobic bacteria in the sludge will denitrify the waste. This is the principle used by batch reactors. As the name implies, batch reactors treat sewage in batches. A batch of sewage is allowed to settle so that solids are removed; the batch of sewage is then aerated and mixed and then allowed to settle for a period of anaerobic treatment (this process may be repeated several times on the same batch). When treatment is complete, the finished batch of sewage is pumped out and the next batch enters the unit to begin treatment.

The Town of Exmore is utilizing the batch treatment process to treat the sewage from its town.  The process is commonly known as a Sequencing Batch Reactor (SBR) and can also be used where municipal and industrial wastes require conventional or extended aeration activated sludge treatment. They are most applicable at flow rates between 3000 gpd and 5 MGD but lose their cost-effectiveness at design rates exceeding 10 MGD (USEPA, 1992). Sequencing batch reactors are very useful for the pretreatment of municipal, industrial wastes and small flow applications where little operation attention is available. The Fluidyne Corporation manufactured the Town of Exmore’s wastewater treatment system. The system is a Fluidyne ISAM^TM, a process of activated sludge treatment. Fluidyne has taken the Sequencing Batch Reactor to the next step in treating wastewater. This process incorporates a constant level anaerobic selector chamber, followed by a surge/anoxic/mix (SAM) Chamber, and a Sequencing Batch Chamber (SBR). The figure below shows the key components of the Fluidyne ISMA^TM Sequencing Batch Reactor.

 The Key components of Fluidyne ISMA^TM Treatment Process:

1.                  Anaerobic Chamber

2.                  Surge/Anoxic/Mix (SAM) Chamber

3.                  Sequencing Batch Reactor (SBR) Chamber

4.                  Jet Aeration

5.                  Recycle Line

6.                  Decanter

Sequencing Batch Reactor

A sequencing batch reactor is a modified conventional continuous-flow activated sludge treatment system. The modified conventional activated sludge systems treat wastewater in a series of separate tanks. Sequencing batch reactors carry out aeration and sedimentation/clarification simultaneously in the same tank. They are designed for the removal of biochemical oxygen demand (BOD) and total suspended solids (TSS) from typical municipal and industrial wastewater at flow rates of less than 5 MGD. Modification to the design of the basic system allows for nitrification and denitrification and for the removal of biological phosphorus to occur.

The sequencing batch reactor is particularly suitable for small flows and for nutrient removal. Sequencing batch reactors can be either used for new developments or connected to existing septic systems. Small reactors can be sited in areas of only a few hundred square feet. While sequencing batch reactor cost and operation and maintenance requirements are greater than those for conventional On-site Sewage Disposal System (OSDS), sequencing batch reactors may be suitable alternatives for sites where high-density development requires adequate treatment of wastewater effluent.

 Sequencing Batch Reactor Process Cycle

The operating principles of a batch activated sludge process, or SBR, are characterized is five (5) discrete periods:

1.                  Anoxic Fill

2.                  Aerated Fill

3.                  React

4.                  Decant

Anoxic Fill

The influent wastewater is distributed throughout the settled sludge through the influent distribution manifold to provide good contact between the microorganisms and the substrate. The influent can be either pumped in or allowed to flow in by gravity. Most of this period occurs without aeration to create an environment that favors the reproduction of microorganisms with good settling characteristics. Aeration begins at the beginning of this period.

Aerated Fill

Mixed liquor is drawn through the manifold, mixed with the influent flow in the motive liquid pump, and discharged, as motive liquid, to the jet aerator. This initiates the feast period. Feast is when the microorganisms have been in contact with the substrate and a large amount of oxygen is provided to facilitate the substrate consumption. Nitrification and denitrification occurs at the beginning of this stage. This period ends when the tank is either full or when a maximum time for filling is reached.

Settle

Aeration is discontinued at this stage and solids separation takes place leaving clear, treated effluent above the sludge blanket. During this clarifying period no liquids should enter or leave the tank to avoid turbulence in the supernatant.

Decant

This period is characterized by the withdrawal of treated effluent from approximately two feet below the surface of the mixed liquor by the floating solids excluding decanter. This removal must be done without disturbing the settled sludge.

The availability of artificial intelligence has now made the option of a SBR process more attractive thus providing better controls and results in wastewater treatment. This is coupled by the flexibility of a SBR in the treatment of variable flows, minimum operator interaction required, option for anoxic or anaerobic conditions in the same tank, good oxygen contact with microorganisms and substrate, small floor space, and good removal efficiency. Sequencing batch reactors operate by a cycle of periods consisting of fill, react, settle, and decant. The duration, oxygen concentration, and mixing in these periods could be altered according to the needs of the particular treatment plant. The many advantages offered by the SBR process justify the recent increase in the implementation of this process in industrial and municipal wastewater treatment.

 Advantages

1.         Equalization, biological treatment, and secondary clarification can be achieved in a single reactor vessel

2.         Operating flexibility and control

3.         Minimal footprint

4.         Potential capital cost savings by eliminating clarifiers and other equipment

 Performance

The performance of SBRs is typically comparable to conventional activated sludge systems. Depending on their mode of operation, SBRs can achieve good BOD and nutrient removal. For SBRs, the BOD removal efficiency is generally 85 to 95 percent. SBR manufacturers will typically provide a process guarantee to produce an effluent of less than;

            1.       10 mg / L      BOD

2.       10 mg / L      TSS

3.       5 - 8 mg / L   TN

4.       1 - 2 mg / L   TP

Geoflow Drip Irrigation

Drip irrigation (sometimes called trickle irrigation) works by applying water slowly, directly to the soil. The high efficiency of drip irrigation results from two primary factors. The first is that the water soaks into the soil before it can evaporate or run off. The second is that the water is only applied where it is needed, (at the plant's roots) rather than sprayed everywhere. Drip systems are simple and pretty forgiving of errors in design and installation.

This technology is a specific application of Pressurized Leach Field Dosing. From the dosing tank, filtered effluent is distributed close to the surface of the ground, through small plastic tubing with holes every one or two feet. Leachate is preferentially taken up and processed by the surface planting, and evapotranspiration is a significant factor. According to the National Small Flows Clearing House (NSFCH), which uses the term Low Pressure Pipe (LPP) for this system, it originated in North Carolina and Wisconsin as an alternative to conventional soil absorption systems to eliminate problems such as clogging, mechanical compaction of the soil, and anaerobic conditions due to continuous saturation or a high water table.

Drip "irrigation" is the shallow, slow, pressure-dosed release of pretreated wastewater directly at or above the surface of the soil. Common to all pressurized dosing systems is the uniform distribution of effluent and periodic dosing and resting cycles. Particular to drip irrigation or LPP systems is the use of small diameter piping with underground drip emitters placed about 6" under the surface. Effluent must be treated and filtered before distribution. Effluent is applied at a controlled rate in the plant root zone, which tends to minimize percolation and enhance evapotranspiration (the evaporation of water from soils, plants and surface waters). The level controls in the dosing chamber are set for a specific pumping sequence, depending on the design, which might be as seldom as once a day. The laterals are placed in narrow trenches that allow enough storage volume so that the depth of the wastewater does not exceed 2 or 3 inches of the total trench depth during each dosing cycle. Hydraulic loading rates may vary between 0.01 and 0.4 gallons per day per square foot.

Below is schematic of a Drip Irrigation System.

Process monitoring

There have been several quality control processes employed by the Town of Exmore Treatment Plant to ensure for an efficient operating system below are what has been employed:

            1.      Envircorp Labs out of Harrington, Delaware has been selected to perform all Health Department parameters.

2.      Daily monitoring of the Wastewater Treatment Plane has been established.

3.      Emergency call-in procedures have been established. The automatic dial system will contact 3 key personnel and will contact the Northampton County Sheriffs Department. The system is setup to call if there is an alarm indicating a problem.

4.      Operator is on duty 5 days per week.

5.      A contact person has been established to call for any technical assistance that should arise.

6.      Several daily work sheets have been prepared to record all information. Attached are the current work sheets.