What is urban waste or waste water?
Urban waste water is residential water or a mixture of domestic with industrial waste water and/or rain water. This waste water comes from houses and various other activities (schools and universities, public businesses, workplaces, tourist units, hospitals, laboratories and medical centres, craft enterprises etc).
Domestic waste water is waste water from residential and service areas mainly derived from the human metabolism and commercial activities.
Industrial waste water is any waste water discharged from buildings and areas used for any commercial or industrial activity and which is not domestic waste water or rain water.
Why is it necessary to collect and process urban waste water?
• Prevention of problems during the emptying and maintenance of sewers and septic tanks
• Prevention of odours, environmental pollution
• Protection of the little ground water and aquifers
• Prevention of nuisance when emptying sewers, areas of contamination
• Prevention of pollution of coastal areas and the sea at the discharge points
• Prevention of the impact on public health
• Quality of life compromised
• Compliance with Community Directive 91/271/EEC on the treatment of urban waste water (communities >2000 e.p)
• Implementation of the Sewage Systems laws 1971-2007 (L 148(I)/2007).
Main advantages of the treatment of urban waste water
• Protection of public health
• Protection of the water resources and the environment in general
• Exploitation of the tertiary treated water made available for irrigation in agriculture and in green areas and consequently, saving of drinking water
• Exploitation of the produced sludge as a soil improver in agriculture
• Exploitation of biogas as a renewable source of energy by the production of electricity.
Possible methods for the treatment of urban waste water
• Aerobic Treatment of Waste Water: purification of waste water with the assistance of aerobic microorganisms suspended under aerobic or anoxic conditions.
• Processing with activated sludge: it is the process for the biological treatment of waste water in which the mixture of waste water and active sludge is stirred and aerated. Subsequently, active sludge is separated from treated waste water. Part of the active sludge is removed from the system as redundant mud. Some kinds of processes with active sludge are the following:
- Batch reactor
- Gradual aeration
- Oxidation ditch
- Extended Aeration
- Contact and stabilisation
- Treatment of waste water in attached growth such as trickling filters, fixed granular beds.
- Anaerobic treatment of waste water: purification of waste water with the assistance of microorganisms under anaerobic conditions, applied on industrial or livestock waste.
- Finally, there are presently new technologies initially using active sludge followed by special membranes for the separation of active sludge from treated waste water.
Parameters defining urban waste water
• BOD5: Biochemical oxygen demand
• COD: Chemical oxygen demand
• SS: Suspended solids
Usual stages for the Treatment of Urban Waste Water
Pre-treatment
At this stage, the large particles are removed such as stones, sand, gravel, wood, plastic items, greases and oils. As soon as it enters the Waste Water Treatment Plant, waste water goes through the stage of pre-treatment, which is usually carried out through grids (1-25 mm), a sand collector and grease traps.
Primary treatment
This stage is the treatment stage which pre-requires the removal of suspended solids from waste water or the stage after pre-treatment. To remove suspended solids, natural and chemical methods are used. A natural method is the use of settling tanks where as waste water passes through, the majority of suspended solids settle and in this way, suspended solids are removed from waste water. Moreover, at this stage, the BAO of incoming waste water may be reduced by at least 20% before discharge and the total load of suspended solids in incoming waste water may be reduced up to at least 25-50%.
Secondary Treatment
This stage is the stage of treatment with biological processes such as active sludge or other (even non biological processes which may result in equivalent results). During this stage, the removal of the BAO by at least 70% is achieved and the removal of COD by at least 75%. In particular, waste water is directed to aeration tanks where it is stirred and aerated with active sludge. Following that, the above mixture is directed with gravity to the secondary settling tank, where the active sludge is separated by way of natural sedimentation and treated waste water is directed to tertiary treatment.
Tertiary treatment
Tertiary treatment is an additional treatment process aiming to further purify treated waste water from the one achieved through the application of primary and secondary treatment, usually carried out by the infiltration and disinfection of treated waste water.
In particular, at the infiltration stage, treated waste water passes through filters containing various layers of sand of different sizes, to remove the small suspended solids. At this stage the BAO is also reduced.
At the disinfection stage, the treated waste water is chlorinated in the chlorination tank so that the activity of pathogenic microorganisms is reduced under a specific level.
Further treatment
Further treatment includes additional treatment processes of waste water so as to remove nitrogen and/or phosphorus and/or any other pollutant affecting the quality or a specific use of treated waste water, such as microbiological pollution, colour etc. In such case, the following minimum outputs must apply: organic removal of pollution at least by 95% for the BAO and 85% for the COD and at least one of the following:
• Removal of nitrogen at least by 70%
• Removal of phosphorus at least by 80%
• Infiltration
• Microbiological removal achieving a density in E-coli less than 1000 in 100 ml.
These additional processes for the treatment of waste water may be achieved by way of biological processes at the aeration stage and/or even with chemical methods.
Treatment and disposal of sludge
Sludge produced during the biological treatment must be treated so that it may be disposed for agriculture. Such treatment may include thickening of the sludge, stabilisation, addition of chemicals, and dehydration with centrifuge or with belt filter presses, drainage in special drainage areas where it is kept for some time before its disposal for agriculture.