General water and sewage treatment equipment description and operation

There are many ways to treat water to be used in industrial processes and for human consumption. In addition; there are also many ways to treat sanitary sewage so that it can be safely returned to the natural water cycle. Following is a discussion of some of the equipment and systems available to treat water and sewage.

1. Water disinfection. Chlorination is the traditional disinfectant used in municipal water treatment. It is a strong oxidizing agent, inexpensive, reliable, easy to use and monitor and safe when handled properly. Chlorination with chlorine gas is the oldest method of continuous disinfection method used in public water supplies. It was initially introduced in 1904. Disinfection by chlorination has been studied extensively and is the standard by which other disinfection procedures are judged. Disinfecting forms of chlorine are hypo chlorites, chlorine dioxide and products of chlorine-ammonia reactions.
   1. Simple diffuser systems are adequate for distributing chlorine into water, with warming of the supply tank required for high feeding rates. Hazards of working with chlorine include explosions of pressure vessels (especially if corrosion weakens them) and violent reactions when chlorine comes in contact with oxidizable substances.
   2. Chlorination is the final step for most wastewater treatment plants. In addition to disinfection of the effluent, BOD is reduced because reaction with chlorine substitutes for reaction with O2. The residual chlorine discourages iron bacteria that form slimes in effluent conduits and insects are also killed. Most smelly compounds in treatment plant effluents are easily reacted with chlorine, usually to odorless products. Unfortunately, the products of chlorination may be hazardous, and some are known carcinogens. Concentrations in the environment may be much higher than in the effluents because microorganisms or other life forms ingest, and store chlorinated organic compounds.
   3. O3 (ozone) is also used for disinfection. O3 is a colorless gas at room temperature, and has a peculiar, pungent odor. O3 is unstable and cannot be produced and transported. It is generated at its point of use by an electrical corona discharge or UV irradiation of dry air or O2. O3 can be injected or diffused into the water supply stream.
   4. Advantages of O3 over chlorine include the following.
      1. Safety problems of chlorine storage, handling, and transportation are eliminated. O3 is produced on-site.
      2. O3 destroys both bacteria and viruses, while chlorine is not very effective against viruses.
      3. O3 has shorter treatment times (one to ten minutes for O3 versus 30 to 45 minutes for chlorine).
      4. There are lesser pH and temperature effects with O3.
      5. High dissolved O2 concentration from ozonation improves receiving stream quality.
      6. No toxicity to aquatic life has been found in studies of O3 disinfection.
      7. No buildup of bioaccumulatable residuals has been observed in O3-treated effluents.
      8. There is no increase in total dissolved solids in O3-treated water.
      9. Wastewater quality improvements such as turbidity reduction and effluent decolorization accompany O3 treatment.
   5. The disadvantages in using O3 to treat water include the following.
      1. It is costly to produce O3, for both capital equipment and operating power requirements.
      2. O3 is toxic. The Public Health Department has set the maximum safe working concentration at 0.1 ppm.
      3. There is great difficulty in accurately determining the concentration of O3 in water. The best method thus far has an error of ± 1 percent.
   6. UV light is another method of disinfection. UV light used for disinfection occupies the spectral range from just below visible light to soft X-rays. UV radiation at about the center of the range has been found to kill or deactivate many pathogens. UV treatment does not necessarily kill the target organism, instead the radiation alters cell DNA so that the organism is sterilized. This serves to inactivate the pathogen so that it cannot proliferate and cause disease.
   7. Bacteria are the easiest group to treat and differ the least in amount of radiation required. Viruses are most resistant and variable. Cysts and worms are unaffected by UV light so if present they must be treated by another procedure.
   8. UV treatment adds nothing to the water and does not require the addition of treatment materials as long as the system used is maintained in good operating condition. Extensive contact time is not required in this process making it a time efficient treatment option.
   9. The major disadvantage is that there is no residual for treatment beyond the device. If contaminants enter after treatment, another disinfection method such as chlorination must be used to sanitize the system and treat the water. Some pathogens deactivated by UV light may be reactivated when exposed to O2. UV light is easily absorbed by solids, including particulate matter in the water or deposits on the lamp surface. As a consequence, UV light treatment should only be attempted on clear water. Water systems which store potable water for long periods may require disinfection to control the growth of biological contaminants and algae. Water samples taken are tested to determine the amount and strength of hypochlorite solution treatment needed. Disinfection can be accomplished by directly injecting hypochlorite into the water, adding hypochlorite into a recirculating side stream or a combination of both.
2. Process water treatment. Open process water systems in cooling towers are constantly exposed to air and with constant evaporation, raw water is continually being introduced. As a result, the cooling water can contain large amounts of dissolved O2 and the concentration of impurities/dissolved solids increases significantly over time. Frequent water quality monitoring is needed to control corrosion, scale formation, growth of biological agents and pH. The dissolved solids are usually removed with controlled periodic blowdown of the system. Treatment methods can be as simple as an operator adding slugs of chemicals to a pot/tank or can be as sophisticated as using a continuous analyzer to automatically control operation of chemical metering pumps. The pumps inject chemicals into the water stream to maintain uniform water quality. The analyzer reads output signals from instruments/probes in the process water stream to measure pH, conductivity, dissolved solids concentration, raw water flow and corrosion. The analyzer signals metering pumps to inject appropriate quantities of chemicals in the water to adjust quality. In less sophisticated systems where periodic sampling has indicated changes to water quality are slow and fairly constant, fixed doses of chemicals can be injected by metering pumps run on timers.
   1. Steam boilers above 600 psig may have problems with water foaming and caustic embrittlement of metal components. For these boilers water quality is typically accomplished by a combination of chemical treatment, deaeration and blowdown.
   2. Deaeration is the removal of dissolved gaseous carbon dioxide (CO2) and O2 from supply water. These gases greatly increase corrosivity and when heated in boiler systems combine with water to form carbonic acid. Removal of O2, CO2, and other non-condensable gases from boiler feedwater is vital to boiler equipment longevity as well as safety of operation. Carbonic acid corrodes metal, reducing the life of equipment and piping. It also dissolves iron which when returned to the boiler precipitates and causes scaling on the boiler tubes contributing to reduced life and also increased energy consumption to achieve heat transfer.
   3. Mechanical deaeration is typically utilized prior to the addition of chemical O2 scavengers. In mechanical deaeration boiler feedwater is heated with steam. This scrubbing action releases O2 and CO2 gases which are then vented. Trace O2 is removed with a chemical O2 scavenger such as Na2SO3 or N2O2. Free CO2 can be removed by deaeration, but this process only removes small amounts of combined CO2. The majority of the combined CO2 is removed in the steam, subsequently dissolving in the condensate and causing corrosion problems. These problems can be controlled through the use of volatile neutralizing amines or filming amines.
   4. Water softeners are used to remove dissolved solids for reduced foaming and scale formation. Water softeners contain a plastic bead or zeolite in a column. The zeolite is saturated with sodium chloride, salt. When water is passed through the column, the calcium and magnesium in the water is replaced with sodium. The water is said to be soft at this point. The sodium compounds do not settle out and cause scale or other problems of the hard water. The column is regenerated with a strong salt/brine solution and back flushed.
   5. Ion exchange units have a cation exchange column to remove metals and hardness and an anion exchange column to control alkalinity and reduce corrosion, embrittlement, hard scale and foaming. Each exchange column uses a treated resin bed to collect contaminants through a chemical exchange process. When the beds become saturated, they are back washed, treated with an electrolyte, rinsed and placed back in service. Duplex columns are used to achieve continuous operation.
3. Water treatment piping. For water treatment systems common pipe, valve, and pump materials include cast and forged carbon and stainless steel. Low pressure systems can use plastic materials such as polyvinyl chloride (PVC).
4. Sewage treatment Other methods for treating sanitary sewage are discussed in the following paragraphs.
   1. The plug flow activated sludge process is another process used to treat sanitary sewage. This process meets secondary treatment effluent limits. The process includes a bar screen as preliminary treatment and a comminutor (a device which reduces material to minute particles, pulverizer), a grit chamber and oil and grease removal units. The primary aerated wastewater and acclimated microorganisms are aerated in a tank. Flocculent activated sludge solids are separated from the wastewater in a secondary clarifier. The clarified wastewater flows forward for further treatment or discharge. A portion of the clarifier sludge is returned to the aeration tank for mixing with the primary-treated influent to the basin and the remaining sludge is pumped to the sludge handling portion of the treatment plant.
   2. Another sanitary treatment process is the stabilization or oxidation pond process. This process uses a relatively shallow body of wastewater in an earthen basin to treat a variety of wastewater and functions under a range of weather conditions. The ponds can be aerobic or layered with aerobic and anaerobic layers. They can be used in combination with other treatment processes. Their operational and maintenance requirements are minimal.
   3. Advanced wastewater treatment achieves pollutant reductions by methods other than sedimentation, activated sludge and trickling filters used in conventional treatment. Advanced treatment employs a number of different unit operations, including ponds, post-aeration, micro-straining, various types of filtration, carbon adsorption, membrane solids separation, land application, biomass growth, soil biota growth, nitrification/de-nitrification and other treatment processes. Phosphorus and nitrogen removal processes can consist of additional treatment ponds; post-aeration through advanced methods; and the addition of minerals, lime, metal salts and polymers for removal through flocculation (mass formed by the aggregation of a number of fine suspended particles) or precipitation.
5. Sewage treatment ancillary equipment. Following is a description of some of the types of ancillary equipment needed for the treatment of sewage.
   1. Sewage flow measurement is needed to assure compliance with permits and to evaluate and adjust treatment processes. Flow can be measured with weirs, Parshall flumes and magnetic and ultrasonic flow meters.
   2. Sewage sampling is needed to assure compliance with permits and to evaluate and adjust treatment processes. Proportional flow, composite and grab-sample collection sampling is done at several locations in the process for this purpose.
   3. Monitoring equipment is used to indicate and/or record flow quantities and pressure, temperature, liquid levels, velocities, dissolved O2, biochemical O2 demand, total suspended solids, ammonia, nitrate and pH.
   4. Sewage lift stations and sump pumps are needed where there is not enough elevation drop available for the sewage to flow by gravity all the way to the septic tank or treatment plant. Lift stations provide pits or sumps with submerged centrifugal motor driven pumps or compressed air driven ejector pumps.