WASTEWATER TREATMENT Wastewater treatment is expected to avoid or minimize environmental degradation or impairment of health. Municipal wastewater convey's a variety of wastes and normally consists of about 99.9% water and about 0.1% solids. This Authority serves private dwellings, commercial establishments and industrial contributors that produce between 70 to 120 gallons per day per person.
OBJECTIVES OF WASTEWATER TREATMENT
The primary objective of this wastewater treatment plant operation is to meet the permit requirements. At the same time, the operation must protect the safety, health and well-being of the plant's employees and neighbors. In establishing the requirements for wastewater treatment, the regulatory agencies considers the following, as well as compliance with minimum statutory requirements:
1. Prevention of disease,
2. Prevention of nuisances,
3. Avoidance of water supply contamination,
4. Elimination of all pollutant discharges to navigable waters,
5. Maintaining clean waters for the propagation and survival of fish and other aquatic life,
6. Protection of waters for personal bathing and recreational use,
7. Preservation of pristine waters for ecosystem protection, and
8. Conservation of water.
Although the effluent quality is usually limited for a specific discharge and watershed, the volume and the physical, chemical and biological characteristics of the treatment plant influent continually change. Some changes are short-term due to seasonal, monthly or daily fluctuations in the quantity and composition of the various communities wastewater this Authority services. Other changes are long-term arising from alterations of the service area's population, social characteristics, economy and industrial production or technology.
WASTEWATER COLLECTION SYSTEM
The wastewater collection system is composed of a network of sewer pipes, structures, devices, equipment and appurtenances for the collection, transportation and pumping of wastewater. Generally, flow through such a system is by gravity or a combination or gravity and pumping where topography does not allow gravity flow alone.
Three principal types of municipal sewers exist. Sewers that receive wastewater from residential, commercial, institutional or industrial sources, together with small quantities of groundwater infiltration and stormwater inflow are called sanitary sewers. Systems that convey stormwater runoff and other drainage while excluding sanitary wastes are called storm sewers. Sewers that convey both sanitary wastes and stormwater are called combined sewers (CSO's). This Authority has sanitary and combined sewers only.
Preliminary treatment of wastewater at the headworks (Degritter Building) of the treatment plant removes materials that might harm the headworks or impair operation of downstream processes. Substances typically removed include hydrogen sulfide, wood, cardboard, rags, plastic, grit, grease and scum. Methods and equipment used to remove these materials are bar screens and a grit chamber.
Following preliminary treatment, primary treatment removes suspended and floating material. Well-designed and operated primary treatment facilities may remove as much as 60 to 75% of the influent suspended solids and up to 20 to 35% of the total BOD5 (Biological Oxygen Demand for 5 days). However, colloidal solids, dissolved solids and soluble BOD5 will not be removed. Therefore, primary treatment must be supplemented by secondary and possibly other advanced waste treatment to attain the required effluent quality.
Secondary treatment reduces the concentrations of dissolved and colloidal organic substances and suspended matter in the wastewater. Generally, secondary treatment reduces suspended solids and BOD5 to a level between 10 and 30 mg/l. Most secondary treatment processes involve biological treatment. Most of the biological treatment processes for secondary treatment can be classified as attached growth or suspended growth systems. Each system relies on an established mixed population of microorganisms in the presence of oxygen and trace amounts of nutrients. The microorganisms consume organic material in the waste to sustain their life processes and to produce new microorganisms. In attached growth systems, the mass of microorganisms affecting treatment are attached to supporting media or the Authority's trickling filters.
The effluent from the attached growth reactors (Trickling Filters) contains high concentrations of biological solids in suspension. Before further treatment, the solids must be removed by means of a liquid-solids separation process. This Authority uses gravity final secondary clarifiers for the separation process.
ADVANCED WASTEWATER TREATMENT
Advanced wastewater treatment is used to reduce the concentrations of nutrients, nitrogen or phosphorous and soluble organic substances to levels below those normally attained through secondary treatment. The treatment requirements for compliance with the effluent limitations in the permit usually influence the type of advanced wastewater treatment processes are selected. This treatment plant uses a biological process of nitrification towers and sand filtration for clarity of the wastewater prior to disinfection.
Disinfection of wastewater treatment plant's effluent inactivates or destroys pathogenic bacteria, viruses and amoebic cysts commonly found in wastewater. Pathogens may cause outbreaks of waterborne diseases such as typhoid, cholera, paratyphoid, bacillary dysentery, poliomyelitis and infectious hepatitis. The demand for reuse of plant effluent for recreation purposes and the requirements to control toxics, including chlorine, to protect aquatic life resulted in a change in disinfection policies regulating disinfection. These changes, in turn, lead to changes in disinfection practices.
Generally, disinfection processes are classified as natural, chemical, physical or radiation. Historically, chemical treatment using halogens, particularity chlorine, has dominated wastewater disinfection practices. Because of the impacts of chemical disinfection by-products (chlorinated hydrocarbons) on water supplies (aquatic life) and the requirement for dechlorination of discharges, other means of wastewater disinfection, such as ozonation and radiation are now being used. This plant utilizes ultraviolet radiation.
SLUDGE AND RESIDUALS MANAGEMENT
Sludge (Biosolids), the settled solids accumulated and separated from the liquid treatment train, must be treated prior to disposal because raw sludge is unstable, putrescible and contains pathogenic organisms. The treatment and disposal of sludge is often the most difficult and costly part of wastewater treatment and the sidestreams resulting from sludge treatment can significantly affect the liquid processes.
TYPES OF RESIDUES
Generally. the following three types of wastewater residues can be characterized by their source in the liquid treatment train:
1. Primary sludges,
2. Biological sludges, and
3. Chemical sludges.
After the above sludges undergo solids treatment processes, sludges such as the following result:
1. Thickened sludges,
2. Digested sludges, and
3. Dewatered sludges.
Wastewater treatment produces other residuals including screenings, grit and scum which are temporarily stored and taken to a landfill for disposal. Primary sludges, which usually range from 40 to 60% of the influent suspended solids, generally have a concentration of 2 to 6% solids when removed from the primary clarifiers. Biological sludges, comprised largely of microorganisms, include suspended growth sludges, such as attached growth sludges from the trickling filters. The concentration of biological sludges generally varies from 0.5 to 1.5% solids. This plant pumps the biological sludges from the secondary clarifiers to the primary clarifiers and mix said sludges with the primary sludges in order to achieve the higher percent solids.
SLUDGE (BIOSOLIDS) TREATMENT PROCESSES
Sludge treatment processes typically consist of the following:
The above processes this Authority uses are thickening, digestion, dewatering and pasteurization.
Sludge thickening follows separation of sludge from wastewater and involves sludge volume reduction by removal of water. Thickening used at this plant are gravity thickening tanks. The solids concentration of thickened sludge usually ranges from 1.5 to 8% solids.
STABILIZATION BY DIGESTION
Digestion, a means of stabilizing sludge, reduces the volatile content and pathogen count, thereby producing a less odorous and putrescible material. At this plant anaerobic digestion is used. Anaerobic digestion results from microbial activity in the absence of free oxygen. Anaerobically digested sludge is relatively nonputrescible (approximately 30 to 60% of volatile solids are destroyed in this process) and the off-gas produced contains about 65% methane. The methane at this plant is collected, stored used as a fuel for the digester boilers and a co-generation system.
Dewatering further reduces sludge volume and weight. Dewatering equipment includes belt filter presses, sand drying beds, vacuum-assisted drying beds, centrifuges, plate and frame filter presses and vacuum filters. Belt filter presses are used at this plant for dewatering. Belt filter presses are used with polymers and can achieve a 15 to 25% solids cake.
PASTEURIZATION (THERMALLY TREATED SEWAGE SLUDGE)
A form of pasteurization, thermally treated sewage sludge is used at this plant. This alternative may be used when the pathogen reduction process uses specific time-temperature regimes to reduce pathogens. Under these circumstances, time-consuming and expensive tests for the presence of specific pathogens can be avoided. It is only necessary to demonstrate that: 1. Either fecal coliform densities are below 1,000 MPN (most probable number) per gram of total solids (dry weight basis), or Salmonella sp. bacteria are below detection limits (3 MPN per 4 grams total solids, dry weight basis) at the time the sewage sludge is used or disposed, at the time the sewage sludge is prepared for sale or given away in a bag or other container for land application, or at the time the sewage sludge or material derived from the sewage sludge is prepared to meet the requirements in EPA 503.10(b), 503.10(c), 503.10(e) or 503.10(f). 2. And the required time-temperature regimes are met (30 minutes at 122 degrees).