Urban Wastewater Management

Water Waste Treatment Process

Typical Treatment Process
© Department of Environment, Community and Local Government

Step 1: Sanitary Sewer                                    Step 4: Aeration Tanks
Step 2: Grit Chamber                                      Step 5: Secondary Treatment Tank
Step 3: Primary Treatment
The image above shows the layout of a typical treatment works. Ideally, wastewater treatment in a municipal treatment works involves four stages: preliminary, primary, secondary and tertiary treatment. There are two end products from the treatment process; sludge solids and liquid effluent. The treatment process reduces the effluent so that it will not adversely affect the quality of the receiving waters.
Preliminary treatment takes large solids and floating debris from the raw wastewater.
Primary treatment separates the smaller solids.
Secondary treatment uses micro-organisms to remove the biodegradable or organic waste.
Tertiary treatment includes nutrient removal and filtration.

Urban Waste Water Treatment Directive

Urban wastewater can be described as a mixture of domestic and industrial wastewater and run-off rain water. The level of treatment wastewaters receive depends on the size of population served. In urban areas wastewater from homes and industry is carried off by a network of pipes and pump stations to a municipal treatment plant. This treatment of the sewage (the contaminated wastewater) involves primary, secondary and tertiary treatment:

solids are separated from the wastewater
dissolved biological matter is converted to solid mass using micro-organisms
solids are then neutralised and reused or discarded
treated wastewater is discharged to receiving waters

The proportion of waste water subject to secondary treatment has increased significantly from 26% between 1998-1999 to 90% in the 2006-2007 period. This is mainly due to the new waste water treatment plants at Ringsend (Dublin), Cork City, Limerick City, Galway City and Dundalk. Furthermore, because of major investment in recent years, construction of secondary treatment facilities at many locations around the country is at an advanced stage. This is expected to deliver significant improvements in the quality of urban waste water discharges.
The National Urban Waste Water Infrastructure Study was published in August 2005. It is the most extensive examination to date of Ireland 's public waste water infrastructure. The study involved the collection, collation, mapping and analysis of urban drainage systems including waste water treatment facilities and an assessment of future waste water requirements.
The Urban Waste Water Treatment Directive is designed to ensure that sewage collection systems are established for domestic and industrial waste waters and that this waste water receives appropriate treatment to reduce its environmental impact before being released into our waterways

Wastewater Treatment Process

Adelaide’s three major metropolitan wastewater treatment plants process more than 250 megalitres of wastewater every day. The plants are located at: Christies Beach Glenelg Bolivar Wastewater comprises a mixture of domestic sewage (waste from household toilets, sinks, showers and washing machines), industrial effluent, occasional run-off of surface water and ground water which has infiltrated into the sewers. Wastewater is 99.99% water, with a small amount of dissolved or suspended solid matter. At our treatment plants the wastewater undergoes a multi-stage treatment process to clean it before discharge or reuse. Preliminary Treatment The first stage of the treatment process uses screens to remove the larger solid inorganic material such as paper and plastics. This is followed by the removal of particles such as grit and silt which are abrasive to plant equipment. Primary Treatment Following preliminary treatment, wastewater is passed through a primary sedimentation tank where solid particles of organic material are removed from the suspension by gravity settling. The resultant settled primary sludge is raked to the centre of the tank where it is concentrated and pumped away for further treatment. Secondary Treatment This next stage is a biological process which breaks down dissolved and suspended organic solids by using naturally occurring micro-organisms. It is called the activated sludge process. The settled wastewater enters aeration tanks where air is blown into the liquid to provide oxygen for mixing and to promote the growth of micro-organisms. The “active biomass” uses the oxygen and consumes organic pollutants and nutrients in the wastewater to grow and reproduce. From the aeration tanks, the mixture of wastewater and micro-organisms passes into a secondary sedimentation tank (also known as a clarifier) where the biomass settles under gravity to the bottom of the tank and is concentrated as sludge. Some of this sludge is recycled to the inlet of the aeration tank to maintain the biomass, hence the name for the process – activated sludge. The remainder is pumped to anaerobic digesters for further treatment. The clarified wastewater is discharged from the secondary clarifier and passes through for Tertiary Treatment. Tertiary Treatment All wastewater treatment plants use disinfection for tertiary treatment to reduce pathogens, which are micro-organisms which can pose a risk to human health. Chlorine is usually dosed into the treated wastewater stream for disinfection. However, Bolivar uses large ponds in which sunlight and other micro-organisms reduce the pathogens. Additional treatment may be required if the treated wastewater is reused for purposes such as irrigation of food crops or where close human contact may result. Tertiary treatment, such as that provided at Bolivar through the Dissolved Air Flotation Filtration (DAFF) plant, produces a much higher standard of treated wastewater suitable for these purposes. The DAFF plant filters and disinfects the wastewater from the ponds allowing it to be used for direct irrigation of crops through the Virginia Pipeline Scheme. Sludge Treatment Sludge collected during the treatment process contains a large amount of biodegradable material making it amenable to treatment by a different set of micro-organisms, called anaerobic bacteria, which do not need oxygen for growth. This takes place in special fully enclosed digesters heated to 35 degrees Celsius, where these anaerobic micro-organisms thrive without any oxygen. The gas produced during this anaerobic process contains a large amount of methane. At the Christies Beach plant it is used to heat the digesting sludge to maintain the efficiency of the process. Elsewhere the gas fuel is used to generate electricity, with the waste heat used to maintain the digestion process. This electricity is used in the plant, reducing our use of non-renewable energy sources. Once the micro-organisms have done their work water is removed from the digested sludge through mechanical means such as centrifuging, or by natural solar evaporation in lagoons. The liquid remaining at the end of the process is usually pumped back into the aeration tanks for further treatment. The stable, solid material remaining, or biosolids, looks, feels and smells like damp earth and makes ideal conditioner for soil. IFAS IFAS (Integrated Fixed-film Activated Sludge) is an innovative treatment process used to reduce nitrogen in existing wastewater treatment plants. IFAS involves introducing small free floating plastic cylinders into the aeration tanks where they provide a large surface area to which biological growths attach, thereby increasing the treatment capacity of the plant.

Wastewater Management in Rural Areas

Management of rural wastewater mostly involves on-site treatment as there is often no interconnecting means of treating numerous residences in the countryside. In Ireland , wastewater from approximately 418,000 dwellings is treated by on-site systems (CSO, 2006). On-site systems can be subdivided into two broad categories: mechanical aeration systems and septic tank systems.
The mechanical aeration systems include biofilm aerated (BAF) systems, rotating biological contactor ( RBC ) systems, and sequencing batch reactor ( SBR ) systems. In these systems micro-organisms feed on organic materials to stabilise them, and reduce biological oxygen demand and suspended solids in the wastewater.
The septic tank system consists of a septic tank followed by a soil percolation area. As an alternative to a conventional percolation area the effluent from a septic tank can be treated by filter systems such as mound or reed beds with sand, peat, plastic or reed filters followed by polishing filters. These polishing filters reduce the level of micro-organisms and nutrients in the wastewater.
In 2009, the EPA issued a Code of Practice for wastewater treatment and disposal systems in single homes seeking planning permission. Nevertheless, none of these on-site systems are currently regulated. Because there is a constant threat of groundwater pollution by a faulty treatment system, urgent action is required at national and local authority level to ensure that all on-site wastewater treatment systems are constructed and maintained in a suitable manner.