Sources of liquid waste
Liquid waste was defined in Study Session 1 as any waste in liquid form. The composition of liquid waste, also known as wastewater, is highly varied and depends principally on its source. In towns and cities, the three main sources are residential, commercial and industrial areas.
1. Liquid wastes from residential areas
In urban areas, the liquid wastes from residential areas are often referred to as domestic wastewaters. These wastewaters come from our day-to-day living and include those from food preparation, washing, bathing and toilet usage.
2. Liquid wastes from commercial areas
The wastewaters from commercial areas: comprising business establishments, shops, open market places, restaurants and cafes – will mostly resemble those from households. This is because only human-related activities are undertaken in such areas, as opposed to other activities such as industrial production.
Effluent from restaurants and cafes may contain high levels of oil from cooking processes but this can be overcome by using a grease trap (Figure) in their outlet pipes. A grease trap consists of a small tank or chamber which slows the speed of effluent flow. In the grease trap, fats, oils and grease float to the top of the wastewater and form a layer of scum that is contained within the tank. This can then be removed and disposed of as solid waste. Relatively clean water exits from the grease trap for disposal.
Figure: Cross-sectional diagram of a grease trap.
3. Liquid wastes from industrial areas
In industrial areas liquid wastes are generated by processing or manufacturing industries and service industries, such as car repair shops. The type of industry determines the composition of the waste. The wastewaters from facilities that make food products will not be harmful to humans, but those from other industries may contain a variety of chemical compounds, some of which may be hazardous (and therefore potentially harmful). Industrial wastewaters which contain hazardous substances must be treated, and the substances removed before the wastewater is discharged to the environment.
4. Storm water
Although not a form of liquid waste in the same way as wastes from residential, commercial and industrial areas, storm water is also a form of wastewater. Storm water can be contaminated with many different types of pollutant such as fiscal matter, soil, rubber from vehicle tire wear, litter, and oil from vehicles.
Types of wastewater
Wastewater comes in 3 main types namely Blackwater, Graywater and Yellow water.
This is wastewater that originates from toilet fixtures, dishwashers, and food preparation sinks. It is made up of all the things that you can imagine going down the toilets, bath and sink drains. They include poop, urine, toilet paper and wipes; body cleaning liquids, anal cleansing water and so on. They are known to be highly contaminated with dissolved chemicals, particulate matter and is very pathogenic.
This is wastewater that originates from non-toilet and food fixtures such as bathroom sinks, laundry machines, spas, bathtubs and so on. Technically it is sewage that does not contain poop or urine. Graywater is treated very differently from Blackwater and is usually suitable for re-use.
3. Yellow water
This is basically urine collected with specific channels and not contaminated with either blackwater or graywater.
A brief review of some suitable technologies is presented here.
Grey Water Management
• It is estimated and experienced that about 75% to 80% of water supplied through piped water supply schemes comes out as grey water.
• The technologies will have to be affordable and manageable at village level and decentralized depending on the locations and spread of habitations.
• They must be environment friendly, low-cost, hygienic, requiring intermediate or low-level construction and maintenance skills available at village level.
• These can be either on-site or off-site methods, depending on village level requirements.
The grey water in a rural area can be broadly classified as:
1. Domestic grey water
2. Community grey water
Therefore, grey water management system can be established at 2 levels.
1. Domestic level.
2. Community level
I. Onsite or community stand post handpumps etc.
II. Offsite via transport through suitable drainage system.
In a rural area, the major quantity of grey water is generated in a home. When this water leaves the premises of the house, it becomes community grey water. Management of grey water at community level is more elaborate as it demands managerial, financial and technological inputs which need elaborate arrangements.
1. Domestic Grey Water Management
• If grey water is managed at source, in a decentralized manner, by each household, it becomes a more appropriate proposition. Of course, availability of area/courtyard/land surrounding the house will be a requirement.
• It will be an approach where there could be Zero Community Waste or Minimum Community Waste.
• If this is to be brought into practice, it would be necessary to provide appropriate technologies at domestic level.
• These technologies should be low cost, simple, easily manageable by the house hold and at the same time hygienic and appropriate from health aspect.
These technologies in order of preference would be
1. Kitchen garden with silt and grease trap and with or without piped root zone system.
If the grey water is clarified before letting it out to the kitchen garden, the garden grows in a better way. This can be achieved by passing water through a very simple device like silt & grease trap.
2. Domestic leach pit
If availability of open land is a constraint and quantity of grey water is more, a domestic leach pit would be a suitable option. This will involve some construction costs for the family.
3. Soak pit
When the grey water quantity is limited and availability of open land is a constraint, soak pit would be a very simple and cheap option for grey water management.
2. Community Grey Water Management
• In very compact habitations, sometimes, with wall to wall houses, it may not be possible to manage grey water at domestic level. In such a situation domestic grey water would be let out from the house. As a result, community grey water would be formed.
• It will have to be collected, channelized and led to open spaces or outside the village for final treatment.
• Various treatment options can be used for managing such grey water.
• In rural areas, in public places like water stand posts, handpumps, public wells, etc., overflow grey water is generated.
• This grey water, which is usually cleaner, will also have to be managed properly.
• Such water can be managed onsite by adopting suitable technologies.
This community grey water management can be done in four components.
A. Preparing village master plan for sullage management
B. On site management for water from public stand posts, wells, hand pumps etc.
• The following options can be used for on site management of grey water from stand post and pumps etc. as stated above.
1. Plantation with intercepting silt chamber.
If open land is available near the water points, plantation with the grey water from these sources would decidedly be a better option.
2. Community leach pit
When open land is not available, community leach pit could be a technological option. This option can also be used at the end of drainage lines on which there are a limited number of households.
3. Soak away system
When the quantity of water is more, soak away channel technology can be used. Functionally, it is like soak pit; however, in this case, the pit is extended lengthwise, extending may be up to 15-20 feet.
4. Root zone system
This is useful for managing grey water onsite. This system can also be useful for moderately limited community grey water from households. Here, a kind of sedimentation cum filter bed is established on top of which, plants like reed etc. can grow. Microorganisms grow in these beds. The outflowing water from this system is well stabilized and pathogen free. It can be used for horticulture, etc.
C. Collection and transport of community grey water.
• It is necessary to install a suitable system for the collection and transportation of sullage upto the final treatment site. Following two systems are usually advised for this purpose:
1. Surface drain
It has been a common practice in many villages. To construct surface drains to carry away the sullage.
2. Small bore sullage carriage system.
This system comprises of underground relatively superficially laid PVC pipe line of small bore, with intervening intercepting tanks at every 200 feet and at corners or bends.
D. Off site management for drain – transported community sullage.
Community grey water- off site management
For final treatment, some options may be as follows:
1. Sullage stabilization pond system
2. Pisciculture ponds
3. Aerated lagoons
4. Effluent treatment plant
In consideration of financial and technological resources at village panchayat level, most appropriate system is “Waste stabilization pond system”.
Waste stabilization pond system
The system has mainly three components.
1. Anaerobic pond
• In this pond, anaerobic digestion takes place.
• This pond has an effective depth of 8-10 feet and has a hydraulic retention time of 1-2 days.
• If soil is very collapsible, a brick lining or stone pitching may be used for this pond.
• The grey water flows into this pond as a first stage.
• The solids from grey water settle down in this pond and gradually get anaerobically digested.
2. Facultative pond
• The settled water from anaerobic pond is led through anaerobic pond outflow pipe system to the facultative pond.
• It is called facultative because in this pond, bottom layer undergoes an aerobic digestion while middle and upper portions have aerobic process in progress.
• In these ponds, the digestive process is a result of three mechanisms.
a) Aeration from air through the surface area
b) Pond bacteria utilize organic matter in the grey matter for their own metabolism.
c) Oxidation due to oxygen liberated from photosynthetic activity of algae due to incident sunlight. Algae grow due to availability of plant nutrients from bacterial metabolism
3. Maturation pond
• The partially stabilized water from facultative pond is led through outflow pipe system to the maturation pond.
• In maturation pond, aerobic action takes place and BOD is highly reduced. Also, destruction of pathogens takes place in this pond.
• The dimensions of maturation pond are like those of facultative pond. The depth is 4-5feet and HRT is 3-5 days. The length of the pond is thrice the width.
• The outflowing water from maturation pond is safe to handle, has low BOD and can be profitably used for agriculture, horticulture, pisciculture etc.
This is a quick review of applicable liquid waste management technologies for rural areas. The choice of technology can rely on the situation at each place. These technologies, if used extensively in rural areas will help change the face of villages.
By – Vineeta Tawney