Sludges are generated through the sewage treatment process. Primary sludges, material that settles out during primary treatment, often have a strong odor and require treatment prior to disposal. Secondary sludges are the extra microorganisms from the biological treatment processes. The goals of sludge treatment are to stabilize the sludge and reduce odors, remove some of the water and reduce volume, decompose some of the organic matter and reduce volume, kill disease causing organisms and disinfect the sludge.
Untreated sludges are about 97 percent water. Settling the sludge and decanting off the separated liquid removes some of the water and reduces the sludge volume. Settling can result in sludge with about 96 to 92 percent water. More water can be removed from sludge by using sand drying beds, vacuum filters, filter presses, and centrifuges resulting in sludges with between 80 to 50 percent water. This dried sludge is called a sludge cake. Aerobic and anaerobic digestion is used to decompose organic matter to reduce volume. Digestion also stabilizes the sludge to reduce odors. Caustic chemicals can be added to sludge or it may be heat treated to kill disease-causing organisms. Following treatment, liquid and cake sludges are usually spread on fields, returning organic matter and nutrients to the soil.
Once water has been abstracted, treated, and disinfected, it is stored in service reservoirs before being distributed.
To provide an unrestricted and economical supply of water, water treatment plants usually utilize pumps and pipelines to deliver water to their customers' doorstep. Because demand for water constantly varies throughout the day together with seasonal variations created, for example, by garden watering, service reservoirs is used to store water avoiding the need for constant variations in pumping rates.
Service reservoirs are constructed and sited on top of hills. Water is normally pumped from the Treatment Works at night to the reservoir thereby making use of cheap electricity. At all times of the day water gravitates to households and businesses through distribution network regardless of whether or not pumping stations and Treatment Works are operating.
It is common for water to be abstracted from a source and pumped to a number of reservoirs strategically placed to supply the local population. In rural areas or areas of exception demand, it is common to boost or transfer water to secondary reservoirs at a higher ground level.
Supply Pipelines
Enclosed pipelines are utilized as the most economical method of transferring treated water in bulk from treatment plants to service reservoirs and onwards to perspective customers.
The pipes must be capable of transferring the required flow, be capable of withstanding internal pressure created by pumping without bursting and when laid under roads be capable of withstanding the crushing loadings imparted by vehicle traffic. They must also be suitable to withstand accidental damage caused by mechanical and manual digging, a common problem in urban areas. Metal pipes must be protected against corrosive action by groundwater and soils.
Supply pipelines are designed to last in excess of 75 years and they range in size from 200mm in diameter up to 1000mm. The selection of pipe size and materials needs to take into consideration many factors. Cast iron pipes were commonly used for over 100 years, whilst today supply pipelines are usually of ductile iron or steel construction.
Most treatment plants are invariably located at low levels while service reservoirs are sited on hilltops to enable water to gravitate into supply. In order to physically move water from treatment plants, it is necessary to pump water uphill to reservoirs.
Booster pumping is used where development has taken place in a local high area, which cannot be satisfactorily supplied from an existing reservoir. The booster pumps are automatically controlled and as the need for boosting is frequently restricted to the hours of peak demand, the pumps may stand idle for some hours each day.
It is normal for pumping times to be scheduled and programmed in advance, or they may be controlled from a centrally manned operations center.
Water that has been treated and suitable for drinking is stored in service reservoirs. Service reservoirs therefore need to be totally enclosed and protected from outside contamination by animals, vegetation or ground and surface waters. They should not be confused with surface water storage reservoirs commonly seen in other parts of the country and used for fishing and other waterborne activities. Reservoirs are generally constructed from reinforced concrete for strength and security.
Service reservoirs are used to fulfill the following functions:
1. To provide a reserve of treated water so as to minimize the possibility of supply interruptions due to failure of mains, pumping or treatment plant.
2. To enable a fluctuating demand within the distribution system to be met.
3. To enable pumps to operate at a constant output and to make economical use of power tariffs.
4. To provide a reserve of water for fire fighting.
5. To provide stable and adequate mains pressure in the distribution network to minimize bursting.
Pipes are used to distribute treated water from reservoirs to users. The water pipes network consists of a vast array of pipes and fittings in different sizes and materials.
Pipes may be sized to ensure excessive energy losses do not cause unacceptably low pressure. Calculations may be done by hand, however sophisticated computer programs are available to 'model' the pipe network. The model may be used to evaluate the impact of increasing water demands and the need for new pipelines. Once a pipe size and theoretical route has been evaluated a suitable pipe material must be chosen.
The most common currently used materials today for new main pipes are ductile iron, polyethylene, unPlasticised Polyvinyl Chloride (uPVC) and steel.
Inevitably leakage can occur from time to time, thus a continuous program of leakage monitoring and detection is necessary. Once the leak location has been pinpointed, the necessary repair must be planned in order to minimize the interruption to water supplies as well as to road users.
Recycled water can satisfy most water demands, as long as it is adequately treated to ensure water quality appropriate for the use.
Recycled water is most commonly used for nonpotable (not for drinking) purposes, such as agriculture, landscape, public parks, and golf course irrigation. Other nonpotable applications include cooling water for power plants and oil refineries, industrial process water for such facilities as paper mills and carpet dyers, toilet flushing, dust control, construction activities, concrete mixing, and artificial lakes.
Although most water recycling projects have been developed to meet nonpotable water demands, a number of projects use recycled water indirectly for potable purposes. These projects include recharging ground water aquifers and augmenting surface water reservoirs with recycled water. In ground water recharge projects, recycled water can be spread or injected into ground water aquifers to augment ground water supplies, and to prevent saltwater intrusion in coastal areas.
Indirect potable reuse refers to projects that discharge recycled water to a water body before reuse.
Direct potable reuse is the use of recycled water for drinking purposes directly after treatment. While direct potable reuse has been safely used in Namibia (Africa), it is not a generally accepted practice in the US.
Generally there are two types of recycled water reuse: direct and indirect.
With direct reuse, treated wastewater is piped into some type of water system without first being diluted in a natural stream or lake or in groundwater. One example is the irrigation of a golf course with effluent from a municipal wastewater treatment plant. Indirect reuse involves the mixing of reclaimed wastewater with another body of water before reuse. In effect, any community that uses a surface water supply downstream from the treatment plant discharge pipe of another community is indirectly reusing wastewater.
Indirect reuse is also accomplished by discharging reclaimed wastewater into a groundwater aquifer and later withdrawing the water for use. Discharge into an aquifer (called artificial recharge) is done by either deep-well injection or shallow surface spreading.
Quality and treatment requirements for reclaimed wastewater become more stringent as the chances for direct human contact and ingestion increase. The impurities that must be removed depend on the intended use of the water. For example, removal of phosphates or nitrates is not necessary if the intended use is landscape irrigation. If direct reuse as a potable supply is intended, tertiary treatment with multiple barriers against contaminants is required. This may include secondary treatment followed by granular media filtration, ultraviolet radiation, granular activated carbon adsorption, reverse osmosis, air stripping, ozonation, and chlorination.
Through the natural water cycle, the earth has recycled and reused water for millions of years.
The water cycle is the simplest natural cycle on Earth. Solar energy evaporates water from the ocean, lakes and rivers. Millions of liters of water rise into the atmosphere as an invisible gas - water vapor. This process is called evaporation.
As the water vapor is pushed over the land by winds and rises over mountains, the water vapor cools and turns back into tiny water droplets, forming clouds. The droplets joining together are termed condensation. These droplets fall to earth as rain (precipitation).
The rain runs into streams and rivers, which eventually flow into lakes or the sea and the cycle begins all over again.
1. Rain, hail, snow, and fogs
2. Interception and evaporation from plant surfaces
3. Depression storage (puddles)
4. Runoff
5. Soil infiltration and percolation
6. Transpiration and evaporation
7. Water table and ground water
8. Creeks, brooks, rivers and lakes
9. Swamps, wetlands, billabongs and anabranches
10. Oceans, seas, harbors, bays and gulfs
Section I: Introduction
Section II: Present (YOU ARE CURRENTLY IN THIS SECTION)
Section III: Future
Complete Table of Contents
Reference