State of Art Paper on Use of Geotextiles in Waste Water Treatment (Research Paper Sample)

State of Art Paper on Use of Geotextiles in Waste Water Treatment
Abstract:
A large section of population in developing countries lives in rural areas. The typical rural settlement is characterized by lack of infrastructure facilities like poor road connectivity, limited access to safe and potable water supply and lack of proper sanitation facilities. Rural sanitation encompasses components like, waste water management, solid waste management, domestic waste management, domestic sanitation practices, personal hygiene and overall village sanitation.
The conventional approach for waste water management, collecting the waste water away from settlement with the help of network of drains and treating it, is not an affordable option in case of rural areas. The onsite management of waste water is an option that has to be explored in detail. It involves use of locally available materials or low cost technical inputs to ensure efficient and effective treatment of the waste water. One of the emerging technologies for on-site domestic waste water management is use of geotextiles. This paper is an attempt to review various technical aspects of use of geotextiles for waste water treatment.
Keywords: waste water, rural sanitation, geotextiles.
Introduction:
In majority of the developing countries, a major portion of the population resides in rural areas. Most of the population living in rural areas does not have access to basic sanitation facilities.According to UNICEF/WHO joint management plan (JMP), 36% (2.5 billion) of the world population lack improved sanitation facilities. Inadequate access to sanitation facilities leads to impoverishment alongwith drop in productivity levels, thereby affecting overall socio-economic development (1). The rural sanitation programme is a total sanitation package that includes, waste management (solid as well as liquid), domestic sanitation and hygiene and overall community sanitation. This paper focuses on the waste water treatment aspects only. Most of the wastewater is generated from local water supply to the community. The water quality gets degraded by domestic, cattle and other uses in the rural areas. Setting up of conventional wastewater treatment plants in rural areas is not an affordable option. On-site treatment processes like soak pits, septic tanks etc. help to reduce the pollutant load in the wastewater making it a good option for recycling other than human consumption.
Composition of wastewater:
On an average, a typical home generates a waste water flow of around 45-90 gal/person/day. As per Corbitt (1989) (2), wastewater is characterized by five major indices viz. suspended solids, biodegradable material, pathogens, nutrients and dissolved inorganic solids. Most of the domestic wastewater generated in rural areas remains untreated and allowed to accumulate at low point in the rural area. The accumulated water may infiltrate the ground water table and pollute the major source of water supply in the rural areas. The pollution of ground water sources with pathogens and other pollutants may lead to epidemic diseases in the community. The important quality indices that are used to assess the wastewater effluent are dissolved oxygen (DO), temperature, turbidity, pH and total dissolved solids (TDS).
Dissolved oxygen (mg/l) is a measure of capability of the water body to support aquatic life. Biodegradation of organic material in the wastewater discharge reduces the oxygen in the water body.
The presence of pathogens in the water body is measured in terms of fecal coliform expressed as colonies/100ml.
On-site wastewater treatment systems:
A typical on-site wastewater treatment system treats the wastewater effluent before it infiltrates the soil or is discharged in the water body. The various treatment mechanisms can be categorized in two major types viz. treatment systems and soil absorption systems (3). The treatment systems comprise of Aerobic treatment units (ATU), Fixed Activated Sludge Treatment (FAST), Recirculating sand filter, Trickling filter, Sequencing Batch Reactor (SBR) to name a few. The soil absorption system comprises of Mound system, sub-surface drip system and peat field.
Of all these on-site treatment systems, the septic tank system is widely used system for individual households or settlement with sparse population (like rural areas). The effluent from septic tank is then discharged into soil absorption field that works on the principle of sub-surface wastewater infiltration system. Septic tanks remove large suspended material that can easily settle in the tank. It acts as an anaerobic bioreactor causing partial degradation of organic wasteunder anaerobic conditions. The effluent from septic tank comprises of pathogenic micro-organisms and nutrients like nitrates, phosphorous etc. This effluent coming out of septic tanks can either be directly subjected to soil infiltration through mound system or sub-surface drip system or is allowed to pass through bio-filters embedded in soil layers that help in decomposition of the organic matter and nutrients in the effluent.
Mound system of soil absorption:
This is one of the simplest and affordable systems to treat the effluent coming out from septic tanks (3, 4). The mound is a layered structure comprising of sand layer (separates the wastewater application layer from groundwater level), absorption area (stores the wastewater before it infiltrates to the ground water thereby absorbing waste material in the effluent), low pressure distribution (system of pipes to uniformly distribute the wastewater over absorption area), geotextile fabric (prevent entry of soil particles in the absorption area thereby reducing chances of clogging in the absorption area) layer of sandy loam (retains some water that can be used for vegetative cover that will be grown over the mound) and top soil layer (which supports growth of vegetation). The mound system purifies the wastewater through three mechanisms viz. filtration, chemical sorption and assimilation (aerobic digestion of wastewater nutrients).
Geotextiles:
Geotextiles are a class of geosynthetics that are used for repair and protection works in infrastructure construction projects (5). They are primarily used for separation, reinforcement, filtration and drainage functions. The use of geotextiles in wastewater treatment processes is a relatively new concept. The material to be adopted for wastewater treatment depends on surface texture, filtration efficiency (pore size), soil particle size, biofilm forming ability and biofilm retentivity. The pore size distribution of the geotextile can be determined by mercury intrusion porosimetry. The performance of the geotextile as an efficient filter also depends on the quality of wastewater. The turbidity of the wastewater is a measure for the efficiency of the textile. Clear water is considered a reference for measuring clogging potential and permeability efficiency.
The geotextiles are usually synthetic fibres. They are classified on the basis of composition (polypropylene, polyethylene and polyester), thickness and length (continuous filament or short staple). The design of the geotextile filter material depends on three main factors, permeability of the geotextile, soil retention and adequate service life performance. The permeability performance of geotextile is influenced by particle size distribution. The particle size distribution of a geotextile is a unique property of a particular type of geotextile. The knowledge of particle size distribution of the geotextile helps in estimation of geotextile permeability.
Hydraulic performance of geotextiles:
The hydraulic performance of the geotextiles is a measurement of its efficiency in treating the wastewater effluent from the septic tank. The efficiency of the operation of geotextile is measured by gradient ratio test (ASTM D5101). This test is an evaluation of soil-geotextile compatibility. The gradient ratio is defined as ratio of hydraulic gradient in the soil-geotextile composite to that in the soil itself. Under ideal conditions, the gradient ratio should be unity. The determination of gradient ratio is crucial to determine the stability and clogging potential of the particular soil-geotextile interaction.
If the gradient ratio is less than one, then a more permeable zone is developed on the u/s of geotextile due to movement of fine particles resulting in smaller head loss. This causes piping failure in the system. If the gradient ratio is greater than one, then clogging of the geotextile takes place due to accumulation of fines in the geotextile openings.
Thus, the gradient ratio test is a measure to check compatibility between particular soil-geotextile combinations. Based on the soil particle size and type, two processes are related to the geotextile performance viz. blocking and clogging. The coarse sized soil particles are related to blocking of the geotextile pores, while the fine particle size is related to clogging of the fabric pores.
The type of soil structure formed at the u/s of the geotextile determines the efficiency of the geotextile treatment process. In case of well graded soils, due to the formation of bridge network, the soil-geotextile interface has increased efficiency in soil particle retention. In case of silty soils or sandy soils with clay, vault network is formed between soil-geotextile interfaces. Both these structures are desirable for capture of suspended solids in the filter voids.
In case a layer of fine grained soils with low permeability is formed over the coarse grained soil sample, the infiltration of effluent water is severely affected by this layer. This phenomenan is called as blinding. Blocking and clogging processes are also related to the type of geotextile fabric used...