|Wastewater - an Introduction
1.0 What is Wastewater
Wastewater is used water and includes substances such as human waste, food scraps, oils, soaps and chemicals. Domestic wastewater includes water from sinks, baths, toilets, washing machines and kitchen. Wastewater from businesses and industries are more complex as water is use for a wide variety of purposes. Wastewater also includes stormwater runoff which contains a lot of contaminants like hydrocarbons washed off from urban surfaces such as roads, parking lots and rooftops.
Domestic wastewater together with discharges from industry and agriculture has an impact on environmental conditions in rivers and coastal waters. Discharges of waste water add to the general nutrient load contribute to eutrophication problems in rivers and coastal waters.
The impacts of discharges of wastewater include the unsightly littering of the rivers, creating foul smells and potential health hazard. Continued pollution may threaten the survival of aquatic life in rivers.
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2.1 Existing Wastewater Treatment System
At present, there are about 74,000 septic tanks in Kuching City. The other waste water treatment facilities include Imhoff tank catering for single building, oxidation ponds at the Kuching Hospital, Kuching Airport and Holiday Inn Damai, There are a few local central wastewater treatment plants using biological treatment with activated sludge acting in extended aeration mode. The most significant is Normah Medical Centre of capacity PE=950. Other treatment facilities include some small package plants based on activated sludge or attached growth technologies.
The drains and rivers act as combined sewers collecting all types of wastewater such as septic tank effluent, grey water from kitchen sinks, baths, etc. as well as liquid industrial wastes such as oil and grease.
2.2 Septic Sludge Treatment Plant
A Septic Sludge Treatment Plant located at Sg. Tengah, Matang was commissioned in the year 2000 with a nominal capacity of 350 m3/day. The plant is designed based on a schedule desludging program of once in two years for all septic tanks within the catchment area of Dewan Bandaraya Kuching Utara, Majlis Bandaraya Kuching Selatan and Majlis Perbandaran Padawan.
3.0 Wastewater Management Studies
Two studies partly dealing with wastewater has been carried out for Kuching. The studies are:
- Sg. Sarawak Environment Control and River Management Study
- Integrated Waste Management System for Kuching
The State Government has also commissioned a Consultant to conduct a Feasibility study of the Wastewater Management System for Kuching commencing on 8Th may 2002. The study is expected to be completed by February, 2003.
The Objectives of the studies are:
- To identify and evaluate feasibility options
- Preparation of Wastewater Management Masterplan
- To identify the critical zones that are the highest contributors of pollutants to Sg. Sarawak
- Preliminary design of Wastewater Management System for critical zones
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The Environment Quality (Sewerage and Industrial Effluents) Regulations 1979 set out two standards of effluent quality, Standard A and B.
Effluent that is discharged upstream of a water supply intake should meet Standard A, while effluent that is discharged downstream has to meet Standard B. For Kuching City, the State Government has decided that effluent discharged into Sg. Sarawak must comply with Standard A even though Kuching is located downstream of the water intake at Batu Kitang.
Effluent from sewage treatment plants need to be sampled at regular intervals and tested to ensure that it meets the required standards. Tests are carried out as part of a monitoring programme to ensure the efficient operation of treatment processes.
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|Wastewater Treatment Technologies
The Science of wastewater engineering stretches only just beyond one hundred years. Within this period, the applied technology has certainly made great strides in promoting environmental protection.
Fixed-film treatment plays an important role in this history, as the first biological mechanism for wastewater treatment. For many years, options like the trickling filter, intermittent filter and contact bed dominated the technology of wastewater treatment. This status has subsequently been assumed by suspended growth process such as conventional activated sludge, modified activated sludge, oxidation ditch, sequencing batch reactor (SBR) etc.
i. Trickling Filter
Trickling Filter process is a typical biofilm treatment process. A trickling filter utilises microorganisms growing on a media for treatment of wastewater. The wastewater is generally dispersed over a media bed, and then flows down through the media where organic matters are absorbed and decomposed.
ii. Activated Sludge Processes
The activated sludge process is the most common process for wastewater treatment. It involves the biological decomposition of organic matter in wastewater by micro-organisms under aerobic conditions. The activated sludge process involves the incubation of microorganisms for purification of wastewater in the aeration tanks and then separation of the microorganisms and supernatant in the sedimentation tank. The mass of microorganisms is called Activated Sludge.
Various modified activated sludge processes have been developed. Some of these processes are:-
- Step Aeration
- Contact Stabilisation
- Extended Aeration
- Oxidation Ditch
- Pure Oxygen Aeration
A Sequencing Batch Reactor (SBR) is an activated sludge type wastewater treatment system that can carry out various treatment operations in one tank. This is in contrast to conventional waste water treatment where waste water flows from one tank to another and each tank performs a specific treatment operation.
The SBR removes organic material and suspended solids, like most conventional activated sludge systems and it can also be used to biologically remove nutrients–nitrogen and phosphorus.
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|Primary and tertiary treatment
Besides the basic processes, wastewater treatment normally involves two other levels of treatment: Primary treatment involving screening for the removal of things such as paper and plastics, grit and grease removal, and the settling and removal of solids. Tertiary treatment includes a secondary treatment process with a further filtering and disinfecting of the effluent, chlorinating, ozonation, ultraviolet radiation are the some methods adopted.
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|Zero Nuisance Concept and Compact technologies
The conventional wastewater treatment plant with its open air concept is always associated with odour problem, noise pollution and aesthetically unpleasant which resulted in decrease in decreased of land value within the vicinity of the plant. This led to the development of the concept of "zero Nuisance" which literally means "can’t see it…can’t smell it…can’t hear it" has developed due to the stigma attached to wastewater treatment plant.
Many modern waste water treatment plants are now adopting "Zero Nuisance" concept while integrating compact technologies in the treatment process. In most cases the plant is either out of sight or fully housed in a building that is blended with the surroundings.
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Wastewater can be purified by natural purification by the ecosystem in the environment. Wetland projects can be created for further treatment of secondary effluent as a low-cost, energy-efficient disposal alternative. This method is suitable for smaller communities with available land.
A wetland created as a treatment facility may also yield other benefits such as enhancement of wildlife habitat and may serve as a recreational and educational park.
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