:: b c m 5 0 3 :: SEWAGE TREATMENT building services & engineering I

:: LECTURE SEQUENCE

:. 1.: introduction :. 2.: objectives of sewage treatment :. 3.: types of sewage treatment:. 4.: individual septic tank:. 5.: sewage treatment plant:. 6.: criteria of sewage treatment systems:. 7.: location of STP:. 8.: STP for urban area:. 9.: STP for small communities:. 10.: performance of sewage treatment:. 11.: standards

:: 1. INTRODUCTION SEWAGE

Liquid waste from 2 types 1. sanitary sewage. 2. stormwater Sanitary sewage generated from homes, institutions and industries.Stormwater generated from rain or melting snow that drains off rooftops, lawns, parking lots, roads and etc.Sewage is characterized in terms of its physical, chemical and biological composition. The main physical, chemical and biological constituents of domestic sewage may be summarized as follows:Physical Properties : Colour, Odor, Solids, TemperatureChemical Constituents : Organic - Carbohydrates, Fats, Oil, Grease, Proteins, and Surfactants Inorganic - pH, Chlorides, Citrogen, Phosphorus, Sulfur Gases - Hydrogen Culphide, Methane, Oxygen

SEWERAGE SYSTEM Network of pipes that carries sewage to a sewage treatment plant

SEWAGE TREATMENTA multi-stage process to improve and purify the water, removing some or all of the contaminants making it fit for reuse or discharge back to the environment.Discharge may be to surface water such as rivers or the ocean, or to ground water that lies beneath the land surface of the earth.Properly treating the wastewater acceptable overall water quality is maintained.

:: 2. OBJECTIVES OF SEWAGE TREATMENT

Sewage treatment is done in order to remove or alter its objectionable properties so as to render it less dangerous or offensive.Following reasons may be taken into account:

To stabilize the sewage without causing odour and nuisance, and without endangering public health

To prevent damage done to receiving waters such as rivers, canals, coastal waters and etc.

Basic wastewater treatment facilities reduce organic and suspended solids to limit pollution to the environment.

TYPES OF SEWAGE TREATMENT SYSTEM

Treat small volumes of waste (from a single household,small commercial + industrialTreat larger volumes of municipal or industrial waste:: 3. TYPES OF SEWAGE TREATMENT

INDIVIDUAL SEPTIC TANK

The purpose of the septic tank is to separate liquids from solids and to provide some breakdown of organic matter in the wastewater. A septic tank is a buried, watertight container made from concrete, polyethylene or fiberglass, with an inlet and outlet pipe.A septic system consists of two main parts-a septic tank and a drainfield. Wastewater flows from the home to the septic tank through the sewer pipe. The septic tank treats the wastewater naturally by holding it in the tank long enough for solids and liquids to separate.

:: 4. INDIVIDUAL SEPTIC TANK The wastewater forms three layers inside the tank. Solids lighter than water (such as greases and oils) float to the top forming a layer of scum.Solids heavier than water settle at the bottom of the tank forming a layer of sludge. This leaves a middle layer of partially clarified wastewater.

INDIVIDUAL SEPTIC TANK

The layers of sludge and scum remain in the septic tank where bacteria found naturally in the wastewater work to break the solids down. The sludge and scum that cannot be broken down are retained in the tank until the tank is pumped. The layer of clarified liquid flows from the septic tank to the drainfield or to a distribution device, which helps to uniformly distribute the wastewater in the drainfield. A standard drainfield (also known as a leachfield, disposal field, or a soil absorption system) is a series of trenches or a bed lined with gravel or course sand and buried one to three feet below the ground surface. Perforated pipes or drain tiles run through the trenches to distribute the wastewater. The drainfield treats the wastewater by allowing it to slowly trickle from the pipes out into the gravel and down through the soil. The gravel and soil act as biological filters.

Typical section of septic tank

INDIVIDUAL SEPTIC TANK

Malaysias most common form of sewerage system used.Provides partial treatment of the sewage that flows into it and needs to be desludged on a regular basis approx. once in 2 years to ensure that it functions efficiently.The maximum amount of sludge that a septic tank can store is approx. one third of its total volume.Individual Septic Tanks is one of the simplest forms of sewage treatment and dates back to the sewerage system development in France in 1860An IST comprises two chambers connected in a series.In the first chamber, solids from the incoming sewage settle forming a "sludge", while greases and oils float to the surface forming a "scum" layer.Effluent from between the scum and sludge layers then passes into the second chamber where further sedimentation occurs. Finally, the effluent leaves the second chamber and is discharged into a drain or allowed to percolate into the soil.

The sludge in the tank undergoes anaerobic digestion and is converted into more stable organic compounds and gases such as carbon dioxide (CO2), methane (CH4) and hydrogen sulfide (H2S).ISTs are usually designed for a 24-hour retention time Enough storage capacity is provided so that scum and sludge can be deposited in the tank for up two years after which it must be desludged to keep the tank operating satisfactory.

Process:Raw sewage flows into septic tanks, and solid matter or sludge settles to the bottomOil + grease from sewage floats to the top forming layer of scum. Scum prevents oxygen from dissolving in the sewage and results in anaerobic digestion taking place. Raw sewage has to be retained for at least 24 hours for anaerobic digestion to breakdown the solid matter.

PUBLIC SEWAGE TREATMENT PLANTS IN MALAYSIA In Malaysia extensive use has been made of primary treatment systems such as communal septic tanks and imhoff tanks (chamber suitable for the reception and processing of sewage) and unreliable low cost secondary systems such as oxidation ponds. Imhoff Tanks simple form of sewage treatment plants requiring very little operator skill. There is no mechanical equipment to maintain and operation consists of removing scum, reversing the flow to keep an even distribution of sludge and removing sludge.In Malaysia, 38% of public sewage treatment plants in the country are mechanical plants. These plants operate using mechanical equipment that accelerates sewage break down.INDAH WATER KONSORTIUM (IWK) is responsible for planning and rationalizing the public sewerage facilities to reduce the number of treatment plants using the "multipoint concept" or regionalization.

:: 5. SEWAGE TREATMENT PLANT

Public Sewage Treatment Plants in Malaysia

SEWAGE TREATMENT METHODS

Sewage treatment methods may be classified into physical unit operationschemical unit processesbiological unit processes

PHYSICAL UNIT OPERATIONSPhysical unit operations are treatment methods, which use the application of physical forces to treat sewage. These include screening, mixing, flocculation, sedimentation, filtration and flotation.

CHEMICAL UNIT PROCESSESTreatment methods in which the removal or conversion of pollutants by the addition of chemicals or by chemical reactions are known as Chemical Unit Processes. These include precipitation, adsorption (to gather gas, liquid, or dissolved substance on a surface in a condensed layer) and disinfection.

BIOLOGICAL UNIT PROCESSESBiological unit processes describe methods, which remove pollutants by biological activity. Biodegradable organic substances are converted into gases that escape to the atmosphere and cell tissue is removed by settling.

OXIDATION TANK

BASIC BIOLOGICAL REACTIONS IN AN OXIDATION TANK

TYPES OF SEWAGE TREATMENT

Historically unit operations have been grouped together to provide various levels of treatment. Preliminary and/or Primary Treatment physical unit operations and is the first stage of treatment applied to any sewage. Secondary Treatment biological and chemical unit processes.Tertiary treatment combinations of all three.

PRELIMINARY SEWAGE TREATMENT- Preliminary sewage treatment is defined as the removal of sewage constituents that may cause maintenance or operational problems with the treatment operations. - This includes screening and comminution (grinding) for the removal of debris and rags, grit removal by sedimentation and flotation for the removal of excess oil and grease.

2.PRIMARY SEWAGE TREATMENT- In primary treatment screening and sedimentation remove some of the suspended solids and organic matter. - The effluent from primary treatment will contain high amounts or organic matter.

SECONDARY SEWAGE TREATMENT- Secondary sewage treatment is directed at the removal of biodegradable organic and suspended solids, mainly using biological unit processes. - Disinfection may be included in secondary sewage treatment.

TERTIARY SEWAGE TREATMENT - Tertiary sewage treatment includes the removal of nutrients, toxic substances including heavy metals and further removal of suspended solids and organic. - Effluent from tertiary treatment is of a high standard and suitable for reuse.- However, there is no plan to build tertiary treatment systems in Malaysia the focus has been providing a basic standard of preliminary, primary and secondary treatment.

SEWAGE TREATMENT PROCESS

PRIMARY + SECONDARY SEWAGE TREATMENT PROCESS

:: 6. CRITERIA OF SEWAGE TREATMENT SYSTEMS CRITERIA OF SEWAGE TREATMENT SYSTEMThe sewage treatment systems should satisfy all of the following criteria:Health- To avoid the spread of pathogenic organism either by direct contact with the sewage or indirectly via soil, water or food (high degree pathogen destruction).Re-use- Yield a safe recycle product preferably in agriculture and aquaculture.Ecological - In those cases when the waste cannot be reused, the discharge of effluent into a surface water should not exceed the self-purification capacity of the recipient water.Nuisance- The degree of odour released must be below the nuisance criteria.Cultural - the method for waste collection, treatment and reuse should be compatible with local habit and social (religious) practice.Operational- skill required for the routine + maintenance must be available locally or can be acquired with only minimal training.Cost- capital & running coast must not exceed the communitys ability to pay.

LOCATION OF SEWAGE TREATMENT SYSTEMThe location of sewage treatment works depends on the following condition:GENERALFar from any existing commercial + residential areaAdequate land available for future expansionShould be well located to minimize impacts on environment + public water supply intake

FLOOD PROTECTIONThe site should be free from flooding

3.SEPARATION FROM PUBLIC WATER SUPPLYShall be away from public drinking water supplies

ACCESS & TRANSPORTATION CONSIDERATIONAdequate service road for maintenance vehicle should be providedsecured

:: 7. LOCATION OF STP

SEWAGE TREATMENT SYSTEMS FOR URBAN AREA

Treatment processes are divided into treatment which utilises oxygen to breakdown organic matter (aerobic) and treatment which doesn't utilises oxygen (anaerobic). The breakdown of organic matter can occur while in suspension (suspended growth) or on the surface of some type of media (attached growth). In addition, processes using ponds are also sometimes used where large areas of land are available. Treatment processes are categorized in this manner as shown in Table 1.

:: 8. STP FOR URBAN AREA

Table 1 Major Biological Sewage Treatment Processes*systems used in Malaysia

SEWAGE TREATMENT SYSTEMS FOR SMALL COMMUNITIESBecause of their size, small communities have traditionally faced the problems of high per capital costs, limited finances and limited operation and maintenance budgets for sewage treatment.Where populations are less dense, the receiving environment is able to cope with lesser level of treatment, often only primary treatment will be provided.However as population increases, these primary treatment systems must be replaced with secondary treatment systems.Sewage treatment plants for small communities are now prefabricated before delivery to site and these are commonly known as "package plants".Package plants only suitable for small communities. Package plants consist of:Aerated lagoonsExtended Aeration SystemRotating Biological ContactorHigh rate trickling filterTreatment processes are categorized in this manner as shown in Table 2.

:: 9. STP FOR SMALL COMMUNITIES

Table 2: Commonly Used Treatment Systems for Small Communities

Aerated Lagoons

Aerated Lagoons are relatively simple plants to operate and maintain. However, they require large land areas and are therefore rarely found in densely populated urban areas.Normally comprises two or three lagoons in a sequence.Sewage has an average retention in the first pond of five days During this time, bacteria consumed the oxygen to breaking down the organic material in the sewage.Effluent is passed into the second pond where the degraded organic matter and sediments are settle out to form sludge.

The effluent may then pass to a third pond for polishing or be discharged to a receiving waterway. The average retention time in the second pond is one day.

Extended Aeration SystemFine Bubble Diffused Air Extended Aeration Systems (FBDAEA) are mechanical secondary treatment systems. To breakdown the organic material in sewage, bacteria require oxygen. This may be introduced by agitation (using surface aerators) or by submerged "diffusers".Sewage entering a plant in passed through primary treatment where coarse material and grit is removed. The sewage then passes to secondary treatment in the form of any aeration tank where it is injected with fine bubbles of air from submerged diffusersSolids in the sewage are held in suspension by the bubbles and bacteria in the sewage break down organic materials. Sewage is held in the aeration tank between 18-24 hours.The effluent with suspended material is then passed into a "clarifier" (sedimentation tank) where the material settles as a sludge. The sludge is drawn off with some being returned to the aeration tank to ensure enough bacteria are present in the tank to continue the process of breaking down newly introduced sewage.

Rotating Biological Contactor

Rotating Biological Contactors (RBCs) are mechanical secondary treatment systemsSewage entering a plant is passed through primary treatment where coarse material and grit is removed. The sewage then passes through one or more RBC units.An RBC unit comprises a series of closely spaced "circular disks" normally made from a plastic material.The disks are partially submerged in the sewage and are slowly rotated through it.The rotating disks support the growth of bacteria and micro-organisms present in the sewage, which breakdown and stabilise organic pollutants.Effluent from the RBC is then passed through final clarifiers where the micro-organisms in suspension settle as a sludge.RBC units are suitable where land is restricted. They are quite and consistently produce a high quality effluent.

High Rate Trickling Filter (HRTF)

The modern trickling filter comprises a bed of highly permeable medium to which micro-organisms are attached.Sewage is percolated or trickled through this media which is made from rocks (2cm to 10cm in size) or specially designed plastic. Rock beds are typically 2 meters deep and are circular.A revolving arm is used to distribute the sewage over the media. Filters under the media drain the effluent and biological solids, which have become detached from the media.Air is circulated back through the drainage system to the media. The effluent from the drain is settled before discharge to the receiving environment.

Trickling Filter

VIDEO

VIDEO ONEVIDEO TWO 12

VIDEO 1 WASTE WATER TREATMENT

VIDEO 2 WHAT HAPPENS WHEN YOU FLUSH?

Factors influencing the performance of sewage treatment:

VELOCITY OF INFLUENT FLOWThe velocity of flow can be reduced by increasing the length of the travel + by detaining the particle longer time in the sedimentation basin.A steadier flow is neede so that the settling process of suspended material can be within not too short / not too long of detention time.

DETENTION TIMETime that septic effluent spends in the septic tank before flowing out to the drain field.Too short time provided not enough materials removed.Too long time provided septic condition may occur (odour).An increase in flow causes a decrease in retention time + corresponding decrease in solids removal.The detention period should generally varies between 12-36 hours; but commonly depending on the type of plant.

:: 10. PERFORMANCE OF SEWAGE TREATMENT

3.TEMPERATUREPrimary settling is efficiently occurred in warm wastewater and may also allow grease and scum to stay in suspension.Warm water is less dense and offers less resistance to gravity settling but however, wastewater water temperature is difficult to control.

4.PLANT UNIT CHARACTERISTICSDesign problems may occur if a unit too small or too large for the average flow, incorrect weir dimensions or poor baffling, leaking valves and etc.Improper weirs of baffles may allow some of the flow to pass through too quickly.

BIOLOGICAL OXYGEN DEMAND (BOD) & SUSPENDED SOLID (SS) Biological Oxygen Demand (BOD) is a measure of the potential for the effluent to remove oxygen from any water that it is discharged into. This is important because removing oxygen from water will kill other things such as fish, invertebrates and plants that might also need to remove oxygen from the water. An obvious visual sign of high BOD are algal blooms; coloured blue or green in fresh water and red in the sea.Malaysia has two standards for treatment of sewage waste; Standard A is for discharge into drinking water catchment areas and Standard B for all other areas, including the sea. As a rule of thumb standard A should be the minimum target everywhere.Standard A criteria are; BOD = 20mg/l @ 20C and Suspended Solid (SS) = 50mg/l

:: 11. ACCEPTABLE STANDARDS FOR SEWAGE TREATMENT

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