Operation of water and sewage treatment systems

A discussion on the operation of the typical potable water supply, process water supply and small- and large-scale sewage treatment systems follows.

1. Potable water system, operation. For both the facility potable water and process water, the water supply to the facility flows through a shutoff valve, strainer, isolation valves, back flow preventer and pressure regulator. The strainer removes particles from the water and can be back flushed for cleaning. Valves are located up and down stream of the pressure regulator and a bypass installed as needed for change out of the regulator. The back-flow preventer incorporates a drain to bleed off water which back flows as a result of back pressure and a malfunction with one of the check valves contained within the unit. This protects the water supply from contamination by preventing facility potable and process water from flowing backward into the water supply.
2. Process water treatment system, operation. An analysis of the water supply and process water in the chilled and heating water systems determines the type of treatment and chemicals needed.
   1. To treat process water, chemicals are fed into a pot feeder and introduced into the process water system by manipulation of isolation valves which allow process water to flow through the pot, mix with the chemicals and flow back into the system.
   2. Manufactured chemical products contain combinations of proprietary chemicals and different forms of generic chemicals to enhance water treatment performance. Water treatment chemical suppliers tailor chemical treatment needs to local water conditions and establish procedures for safe chemical storage and handling. Some basic chemicals include: sodium hydroxide (NaOH) which increases alkalinity, chelants (EDTA, NTA) which control scaling and sodium sulfate (Na2SO3) and Hydrazine (N2H2) which prevent O2 corrosion.
3. Small scale sewage treatment system, operation. In a septic system, sewage from a small facility flows into an underground septic tank. Solids settle out as sludge on the bottom of the tank and are anaerobically digested. Gases produced by digestion are vented back through the plumbing of the facilities through vent stacks in the roof. A scum forms on the top and an almost clear effluent develops in the middle as solids settle and separate. An inlet baffle aids the separation of suspended solids and provides minimal agitation of the sludge for anaerobic digestion. An outlet baffle prevents scum from entering the outlet pipe and clogging the drain field tile. The outlet pipe is connected to a drain field which consists of a perforated, usually plastic pipe or tile, which may run several hundred feet underground depending on the ability of the ground to percolate water. The sloped drain tile is embedded in layers of sand and rock to facilitate percolation and is located below the frost line. Construction of septic systems is monitored by local authorities to insure proper construction and therefore proper operation of these passive sewage treatment systems.
4. Large scale sewage treatment system, operation. In the larger capacity typical sanitary sewage trickling filter process, the preliminary treatment system utilizes bar screens to remove wood, plastic, rags, paper and other large solid objects from sewage influent. Sand and grit which can damage pumps are also removed. A primary sedimentation tank is next in the process. It allows sufficient settling time to permit approximately 50 to 70 percent of the remaining solids to settle out and removes 30 to 40 percent of the biochemical O2 demand (BOD).
   1. Next a high rate trickling or percolating filter provides an environment encouraging aerobic microorganisms to feed on bacteria in the sewage. Sewage is pumped through spray bars and is trickled over stones in an open- air tank. The trickling and the drops falling from one stone to the next provide exposure of the liquid to air, and the rocks provide a surface for microorganisms to live on. These conditions promote aerobic microorganism growth and consumption of sewage bacteria thus removing the remaining BOD and suspended solids.
   2. A secondary sedimentation tank provides residence time to separate biological sludge from the purified water exiting the trickling filter. To destroy pathogenic organisms, the water is disinfected with hypo chlorate, ozone (O3), or ultraviolet (UV) light and discharged to a stream or natural body of water. Periodic unannounced sampling by local authorities insures that the discharge remains within safe and legal limits.
   3. Sludge from the secondary sedimentation tank consists of 90 percent organic matter and 2 to 4 percent solids and is pumped back to the primary sedimentation tank.
   4. Sludge from the primary sedimentation tank is mostly water containing organic matter. It is pumped to a sludge thickener. In the thickener, sludge volume is reduced to half and effluent is recycled to the primary sedimentation tank. The sludge is then pumped to the digestion unit. In the digester, organic matter in the sludge is decomposed by anaerobic bacteria which further breaks down the sludge rendering it inert and eliminating health concerns. Anaerobic bacteria does not require free O2 to decompose organic matter. It does allow separation of solids and water, reduces sludge volume, increases sludge density and produces combustible methane gas in combination with other gases.
   5. Sludge produced in the digester consisting of relatively stable inert organic and inorganic compounds continues on to a sludge dewatering system and then disposal. It can also be sold as soil conditioner. The supernatant liquids (usually a clear liquid overlying material deposited by settling or precipitation) produced are returned to the primary sedimentation tank and the gases are cleaned and stored. The combustible gases can be used to generate heat for local processes or sold as combustible fuel.
   6. Monitoring of sewage flow and effluent discharge is required by the Environmental Protection Agency (EPA) when National Pollution Discharge Elimination System (NPDES) compliance is required as part of the discharge permit.