CHARACTERISTICS OF WASTEWATER TREATMENT PLANTS AND GENERAL PLANT LAYOUT

Saturday, May 8, 2010

:Types of plants

Wastewater treatment plants and processes have been classified as preliminary, primary, secondary, and advanced treatment
A detailed outline of each process is provided in following :

Elements of advanced wastewater treatment

Advanced wastewater treatment encompasses several individual unit operations, used separately or in combination with other processes, to achieve very high overall treatment efficiencies. These processes employ physical, chemical and biological treatment methods.

The objective of advanced wastewater treatment is to improve the removal of suspended solids, organic matter; dissolved solids, and nutrients.
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1. Polishing ponds. Polishing ponds are used to obtain increased organic and suspended solids removal efficiencies up to 20 percent from existing treatment. Treatment by polishing ponds can be aerobic (the biological activity is predominantly aerobic), or facultative (a combination of aerobic and anaerobic biological
activity).
Polishing ponds are also utilized to allow dissipation of chlorine residual to make discharge compatible with shellfish

2. Post-aeration. Post-aeration applies when a certain effluent dissolved oxygen level must be maintained Post-aeration can be achieved by diffused aeration, mechanical aeration, or cascade aeration
.
3. Microstraining. Microstraining is an effective effluent polishing device for removal of additional
suspended solids and associated biochemical oxygen demand. The process consists of physical straining of solids through a screen with continuous backwashing, utilizing a rotating drum to support the screen. Static screens are also used in particular applications

4. Filtration: Filtration is an effective method for achieving additional suspended solids and biochemical oxygen demand removal following conventional treatment processes. Filtration is also very effective as a part of phosphorus removal systems. Filtration can be applied directly to secondary effluents with or without sedimentation pretreatment by chemical addition

5- Adsorption with activated carbon. The primary function of carbon adsorption as a sewage treatment process is the removal of dissolved organics. This process can be applied as advanced treatment to adsorb non-biodegradable organics, or as a secondary treatment replacing conventional biological
treatment. However, certain organics with small or highly polar molecules (e.g., methanol, formic acid, and
sugars) are not removable by carbon adsorption.
6- Phosphorus removal. The basic phosphorus removal process consists of conversion of polyphosphates to soluble forms and then to insoluble forms, and subsequent separation of the insoluble phosphorus forms from the wastewater accomplished through chemical percipitation using lime or mineral
additives such as alum or ferric chloride.. The process basically involves chemical addition, mixing
flocculation, and sedimentation

7- Nitrogen removal. Methods for removing nitrogen from wastewater include air stripping, biological
treatment, and breakpoint chlorination. Biological nitrification-denitrification appears to be the most practical
alternative in most applications at this time. It involves the biological oxidation of ammonia to nitrate
followed by anaerobic denitrifaction, with nitrogen leaving as nitrogen gas. Nitrification can be accomplished
as a single stage combined with the activated sludge process or as a separate stage. Denitrification is a
separate operation and can be of "suspended growth” or "attached growth” configuration. In this stage,
nitrate is reduced to carbon dioxide, water and nitrogen gas following addition of methanol to provide the
carbon source. Suspended-growth denitrification is an activated sludge-type operation with mixing but
without aeration; attached-growth denitrification is a packed column process with attached biological growth

8- Land application. Land application of secondary treatment effluent can be used effectively as a
means of phosphorous and nitrogen removal, biochemical oxygen demand removal, and solids removal. Since
there is not a direct discharge to a receiving stream, land application in many instances is an attractive
alternative for advanced treatment.

Plant site preparation
Site drainage is an important factor in design of wastewater treatment facilities. Capacities of drainage
structures will be designed in accordance with requirements of TM 5-820/AFM 88-5 series. All treatment
units must be protected from surface rainwater by proper shielding and drainage.

Plant layout
1- Arrangement of treatment units. The first step in determining the best arrangement of units is to arrange all units sequentially according to the flow of wastewater through the system. The resulting hydraulic profile for wastewater flow will determine the relative vertical alignment of each of the plant units. Final
arrangement of the units then results from adaptation of site features to the treatment plant's functional and
hydraulic requirements. Allowance must also be made for the area of operation and maintenance of the
treatment units. If sufficient head is available for gravity flow, the hydraulic requirements will control the
plant layout. Greater flexibility in arranging the treatment plant units is achieved with intermediate pumping of wastewater. The treatment plant must operate during emergency conditions such as power failures and also during periods of maintenance work on treatment units. Dual units should be provided in all feasible
cases to provide operational reliability and flexibility.

2- Conduits and pipelines. Conduits and pipes will be arranged in such a manner as to reduce space
and cost requirements. They will be designed to handle the expected maximum flows through the treatment
plant. Design requirements for pipes and conduits are found in TM 5-814-1/AFM 88-11.
3- Bypasses and overflows. Provisions for bypassing individual treatment units will be made so that each unit can be taken out of service without interrupting the plant operation. Bypasses will not be provided for screen, chlorination units, nor other unit process where duplicate units are available. Overflows will be
used to prevent hydraulic overloading of treatment units, especially biological treatment units. Return of flows or temporary storage of wastes not treated or alternate treatment must be provided. Refer to facility discharge permit for limitations on plant component bypasses and overflows.

4- Future expansion and flexibility. The plant designer will consider provisions for expansion by allowing sufficient space for additional units (and additional conduits) to be installed in the future. The plant will be designed so that installation of additional units or repair of existing units will not disrupt operations

5- Treatment plant discharge. Outfall sewers will be extended to the low-water level of the receiving
body of water or to submergence required by regulatory authority to insure satisfactory dispersion of the plant effluent. Provisions for effluent sampling and monitoring are required. The design will assure the structural integrity of the outfall, prevent failure due to erosion, and prevent back-flow during flooding.TM 5-814-3/AFM 88-11,

: Plant hydraulics
1- Hydraulic loadings. The overall head allowances required for various types of wastewater treatment plant.
2-Limiting velocities. A minimum velocity of 2.0 feet per second at design average flow is required for channel flow. At minimum flows, a minimum velocity of 1.5 feet per second is required to prevent suspended solids from settling in flow channels
3- Head loss. The total head loss through a treatment plant is the sum of head losses in the conveyance of wastewater between elements of the treatment process and the losses of head through treatment units
Head losses from wastewater conveyance are due to frictional losses in conduits, bends and fittings, and allowances for free-fall surface and for future expansions. TM 5-814-1/AFM 88-11, gives detailed guidance and charts for computing head losses in pipes and conduits. Head losses through process equipment
are dependent on the specific units and are specified by their manufacturers or by the design engineer. The design engineer must consider hydraulic constraints in the layout and selection of process equipment and configurations. Special cases where dual and multiple pumping is required must receive prior approval by HQDA (CEEC-EB) WASH DC 20314-1000 for Army projects and HQ USAF/LEEE WASH DC 20332 for
Air Force projects
. Plant auxiliary facilities
1- General, A potable water supply will be provided.
Sanitary facilities: toilet, shower and lavatory with
hot and cold water supply, will be provided except for intallations with less than 0.1 million gallons per day
capacity.
.2- Controls and monitoring. The plant arrangement will take into consideration the related control and
monitoring requirements. Laboratory facilities will be provided for conducting the necessary analytical testing
for the purpose of process control and compliance with regulatory or NPDES requirements.
Essay by
Env.Consult. ahmed elshwadfy
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