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Integrated water systems for high density residential/commercial development in urban/periurban areas.

Development Characteristics

The typical source of water supply for stand-alone periurban developments is groundwater developed through individual bores (wells) at the development site. Geohydrologic characteristics of the area are such that sufficient quantities have been available to meet requirements. Some chemical quality characteristics such as arsenic may dictate the need for chemical or a combination of chemical and physical treatment to make it suitable for human consumption.

The quantity of water needed to satisfy all development needs including typical domestic uses ranges from 100 to over 200 L per capita per day depending on the season, income level, landscape design, and water-based amenities. Meeting flow requirements for fire protection will likely be an overriding factor in the design of source and delivery capacity units and the possible use of storage components.

Wastewater is generated in large scale residential units at the rate of approximately 80 L per 100 L of water used. The most common historic plumbing design uses a single sewer pipe system to collect wastewater from all uses including kitchen, bathroom washing and sanitary waste, and laundry wastes. If wastewater is produced in this traditional manner, conventional wastewater treatment will be required to meet elementary disposal requirements. This may also serve as the base treatment level for higher (tertiary) treatment to produce reclaimed water that can be used to meet lower order water supply needs. The degree of tertiary treatment is essentially modular and can be designed to meet specific endues requirements, ranging from non-contact uses including landscape irrigation to other uses potentially involving contact but non consumption uses.

Increasing sophistication in the design of the water use, wastewater collection, and treatment systems may include various levels of segregation of wastewater from kitchen, bathing, and laundry, with independent treatment appropriate to the quality of the wastewater stream, and redistribution to meet lower order supply needs potentially including toilet flushing and outdoor non contact uses.

Integrated Water System Components

Integrated water system components for application to high-rise residential developments area include the basic fresh water supply, the wastewater collection and treatment system, with potential additional tertiary treatment components to provide for at least limited replacement of fresh water needs with reclaimed wastewater. The basic system components are outlined below.

Integrated water system components for application to high-rise residential developments area include the basic fresh water supply, the wastewater collection and treatment system, with potential additional tertiary treatment components to provide for at least limited replacement of fresh water needs with reclaimed wastewater. The basic system components are outlined below.

Water Supply. Development of a bore water supply for residential use consists of locating a water bearing formation producing sufficient quantity of water from a depth from which it is economical to pump. Water from alluvial formations near the ground surface not confined below an impervious layer will not usually be of sufficient sanitary quality to be useable.

Water quality must be established by testing to assure that the water is uncontaminated biologically and that it is of suitable chemical quality. Typically, bore water will be free of suspended solids and if from a secure formation, also free of microorganisms of public health significance. Chemicals that may be present dictating the need for treatment may include iron, manganese, arsenic, and sulfides. If one or more are present they may be removed by well-established treatment procedures including chemical oxidation, precipitation, and filtration. Equipment for such procedures can be either purchased as proprietary catalogue units or fabricated for local installation.

Wastewater Treatment. The treatment of residential wastewater is similar to the treatment of municipal sewage although not complicated by the presence of sometimes difficult waste materials from commercial and/or industrial operations. Domestic wastewater contains waterborne residues of the activities of daily life, washing, cleaning, cooking, and sanitary wastes. Treatment must be designed to remove substances that can or should not be discharged to the environment or would be unacceptable in reclaimed wastewater to be reused.

The principle waste components requiring attention in treatment include the following: 1) large floating materials such as paper, rags, food remnants. These require a screening or efficient grinding to permit their subsequent removal; 2) small floating particles with oils and grease that are lighter than water. These may be removed by skimming to avoid problems in subsequent treatment units; 3) settlable solids which are typically removed in an initial (primary) settling process; 4) dissolved and colloidal organic material that can be decomposed by a biological treatment process; 5) a second settling process is used to remove the biomass that grows in the biological treatment process and is needed for recirculation to make the treatment process work efficiently; and 6) live microorganism including human pathogens that may be of public health significance. The treated effluent is typically disinfected prior to discharge. The rate at which normal human activity produces wastewater is not uniform through a 24 hour daily cycle. In fact it is much less at night requiring treatment tank volumes to be designed to even out flows to permit treatment processes to perform with consistent effectiveness.

Wastewater Treatment Systems

Wastewater treatment systems that are well-adapted to high-rise residential applications are typically based on conventional biological “secondary” treatment that should produce an effluent suitable for discharge to the environment or as the feed for additional treatment to reclaim all or part for reuse. The space limitations of such applications have led to space saving designs incorporating each of the six essential treatment steps in either rectangular or cylindrical formats. Such systems are often referred to as “package plants” owing to their all-inclusive and single unit appearance.

The most common basis for process design for such systems is the “activated sludge” process in the “extended aeration” mode. This approach depends on a high mixed liquor suspended solids (MLSS) concentration in relation to the “food” supply. This results in minimizing the amount of sludge that requires disposal. In recent years many modifications to the basic extended aeration approach have been developed to minimize operational problems and to improve performance reliability. Some of these include use of fixed film media within the aeration tank to enhance organism retention and reduce sensitivity to variations in feed composition. The ultimate modification directed to this aspect of treatment is the membrane bioreactor, virtually eliminating the need for the secondary clarifier. Enhanced solids removal and retention can also be achieved using inclined plate settling components.

Packaged activated sludge systems have been widely applied to treating wastewater from sources ranging from small subdivisions with 50 or even less population equivalents (PE) to small to medium sized communities of 20,000 PE or even more. The range of 1,000 to 5,000 PE likely in high rise residential developments is well suited to the “package plant” approach.

 

 

Examples of cylindrical format and rectangular format package activated sludge plants.

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