Water consumed in daily activities like cooking, bathing, laundry and other household activities ends up in wastewater. This wastewater usually contains pollutants like organic matter (BOD), harmful bacteria, viruses, and chemicals. Exposure to untreated wastewater can lead to outbreaks of waterborne diseases and contamination of the local ecosystem. To mitigate these risks, both urban and rural settings implement engineered sewer system that transports wastewater to centralized treatment facilities or disposal locations, for proper processing and disposal.
Areas lacking Proper Sewer Systems
Sewer systems are not universally available or adequate, particularly in developing regions and challenging terrains. Urban slums and rural villages often lack the facility of a proper sewer system. For instance, one of the world’s largest slums Dharavi, in India, having a population of around 10 million, lacks sanitation and wastewater management facilities1. Similar challenges persist in Kibera in Nairobi, Kenya and Orangi Town in Karachi, Pakistan. Beyond these urban centres, vast rural areas of South Asia and Latin America struggle with limited or non-existent sewer access.
Even in developed countries, geographic and logistical constraints can hinder the implementation of comprehensive sewerage systems. New Orleans, USA, is a prime example of a developed city, facing significant challenges with sewerage systems due to flooding and vulnerability to hurricanes2. The most challenging environments for laying out the infrastructure of the sewer system include mountainous areas, coastal areas, remote areas, islands, deserts, informal settlements etc. Mountainous regions present steep terrain that complicates pipe installation. Therefore, the Himalayan foothills in Nepal, Pakistan and India, the Andes Mountains in Peru and Bolivia, the Appalachian Mountains in the United States and the Alps Mountain range in Switzerland and Austria have limited to no access to centralized sewer systems.
Some coastal areas in developed countries, i.e. California, Florida and Texas in the USA lack sewer systems because they face the challenges of flooding, erosion, and saltwater intrusion. Deserts, with their limited water resources, harsh climate, and challenging soil conditions, pose additional obstacles. Remote islands like Lord Howe Island, Chatham Island, Stewart Island and Christmas Island which are known for their natural beauty lack adequate sewer systems due to geographic isolation. This shows despite widespread misconceptions, the lack of proper sewer systems is not exclusive to underdeveloped countries. Many developed countries also struggle with infrastructure gaps in specific regions.
Onsite Wastewater treatment and disposal techniques for areas lacking centralized sewer system
In areas where a typical sewerage system can’t be used, alternative wastewater management methods, such as pit latrines, soakage pits, septic tanks, and onsite wastewater treatment plants are used. The applicability of these techniques is discussed in the following section, considering the specific scenarios where each method is most suitable.
1. Pit Latrines
Pit latrines are a simple and inexpensive sanitation solution, particularly in regions with limited infrastructure. A pit latrine consists of a hole dug into the ground, often lined with a material like brick or concrete, to collect only human waste. The soil acts as a natural filter, removing some contaminants from the wastewater, while bacteria and other organisms within the waste help decompose organic matter. The depth of the pit latrines varies based on local regulations and groundwater table depth but typically it is dug 1.5-2 meters or 5-6.5 feet deep3.
While pit latrines offer a basic level of sanitation, they are not a sustainable long-term solution. Their limitations include the risk of groundwater contamination, especially if not properly constructed or maintained.
Suitable and Unsuitable Locations for Pit Latrines
Due to their simplicity and low cost, pit latrines are commonly used in rural areas, refugee camps, and emergency situations. Pit latrines are generally not suitable in areas with high groundwater levels, impermeable soil (such as clay), flood hazards, or high population density.
2. Soakage Pit
A soakage pit is a simple open pit dug into the ground. Typically lined with gravel or sand to facilitate water absorption, soakage pits allow wastewater to gradually infiltrate the surrounding soil. Suspended particles and biochemical oxygen demand (BOD) carrying particles are filtered out by the gravel and sand lining, where they get naturally biodegraded over time by bacterial action. However, soakage pits are ineffective at filtering out heavy metals, persistent organic pollutants, and pathogens, posing a risk of groundwater contamination if not properly constructed or maintained.
A soakage pit is typically 1 to 3 meters (3.3 to 9.8 feet) deep4. Between the bottom of the soakage pit and the groundwater table (GWT), a minimum distance of 1 meter (3.3 feet) is recommended to avoid groundwater contamination. This distance, known as the percolation zone, varies based on soil permeability, land use, and local regulations. Moreover, a soakage pit should be located at a minimum distance of 30m from a water source
to avoid water contamination5. Soakage pit is one of the cheapest techniques to dispose of wastewater, used in rural villages, and remote areas in both developing and developed countries.
Suitable and Unsuitable Locations for Soakage Pit
Land with loamy, sandy, or gravelly soils, having higher permeability rates, is ideal for soakage pit construction. It is generally not suitable in areas with high groundwater levels, impermeable soil (such as clay), flood hazards, or high population density.
3. Septic Tank
Unlike pit latrine and soakage pit, a septic tank is not a wastewater disposal method, instead, it’s an onsite wastewater treatment method, used in areas lacking access to centralized sewer systems. Septic tanks are underground tanks where wastewater is collected. Solids in the wastewater settle to the bottom of the tank, forming sludge, while the liquid portion, called effluent, floats to the top. During an ideal retention time of 24-48 hours, bacteria present in the sludge break down the organic matter through anaerobic digestion (since air is not present/introduced), producing biogas and reducing the pollutant load (Biochemical oxygen demand). The treated effluent is typically discharged into a drain field or absorption trench. To remove accumulated sludge, the septic tank required annual desludging.
Single-compartment tanks are suitable for small amounts of wastewater, where the entire process takes place in the same compartment. For larger volumes or stricter regulations, 2-compartment septic tanks are preferred. The 2-compartment design separates solids and liquids more efficiently, preventing drain field clogging. In the first compartment (inlet chamber), solids settle, forming sludge, while the liquid moves to the second chamber for further treatment. Septic tank reduces 25-45%6 of the organic load/pollution load and additional compartments can enhance the efficiency of the septic tank.
Septic tanks can be adapted to various sizes, from individual households to small communities. The size of a septic tank varies based on wastewater flow, population, property size, BOD load and local regulations. Generically for a 1m3/day flow with 300 mg/l of BOD, a 2.23m diameter x 1.06m height tank is sufficient. Septic tank, as a sealed underground wastewater treatment system, offers a versatile solution adaptable to various geographic and environmental conditions.
4. Drain Field
A drain field is a wastewater disposal method which is used after the primary level of wastewater treatment i.e. septic tan. The drain field, also known as an absorption trench or leach field, is an underground area where wastewater (mostly effluent of septic tank) is discharged. It consists of a network of perforated pipes laid horizontally in trenches. The pipes are surrounded by a layer of gravel or crushed stone to facilitate the distribution of effluent and promote percolation. Discharged wastewater seeps out of these perforated pipes and percolates into the surrounding soil, which acts as a natural filter, removing contaminants from the wastewater. The filtered water eventually returns to the natural water cycle.
Percolation Rate
The percolation rate is the speed at which water can pass through the soil. A high percolation rate is essential for effective drain field performance. Soil permeability tests can be conducted to determine the percolation rate of a specific site.
Factors Affecting Drain Field Performance
Soil type: The design and performance of a drain field highly depend on the soil type. Sandy soil has high permeability compared to Loamy and clay soil. Therefore, sandy or gravelly soil is ideal for a drain field.
- Groundwater Levels: High groundwater levels can limit the absorption of effluent and increase the risk of contamination. Is some areas where groundwater table is high, use of drain fields is prohibited by local regulatory authorities.
- Slope: The slope of the land can affect the flow of effluent through the drain field.
Design Considerations
Wastewater application rate: The rate varies based on soil type. For sandy and loamy soil, 49 l/m² can be applied daily, while for clay, 9.4 l/m² is suitable. “For 1 m³ of wastewater, a sandy soil requires an area of approximately 20.4 m², while a clay soil would need around 106 m².”
Pipe layout: Pipes in the drain field should be laid in a pattern that allows for even distribution of effluent. The pipes should be buried deep enough to avoid freezing or damage but at least 1 meter above the groundwater table to prevent groundwater contamination.
Drain fields are not suitable in flooded-prone regions, areas with high groundwater levels, steep slopes and clay soil.
5. Constructed wetland
Constructed wetlands are engineered systems designed to copy natural wetlands for both the treatment and disposal of wastewater. They use plants, soil, and microorganisms to naturally filter and purify wastewater, reducing pollutants such as nutrients, suspended solids, and pathogens, up to 80% through biological, chemical, and physical processes7. Wetlands provide a cost-effective solution for secondary and tertiary treatment stages of wastewater treatment.
Capacity
The capacity of a wetland depends on factors like size, depth, hydraulic loading rate, and the type of vegetation used. Typically, wetlands can treat moderate volumes of wastewater, often ranging from 1,000 to 10,000 litres per day per hectare for smaller systems. Larger wetlands can handle greater volumes if designed with the appropriate infrastructure.
Suitable and non-suitable locations
Wetlands are suitable for rural or semi-urban areas where space is available, remote areas without access to centralized wastewater treatment plants, agricultural zones where treated water can be reused for irrigation, and industrial sites generating non-toxic wastewater that can benefit from natural treatment processes. Wetlands are particularly suitable in regions with a warm climate and where the terrain allows for natural water flow and evaporation, enhancing the treatment process.
Locations with limited land availability, cold climates, high groundwater tables, heavy rainfall or flood-prone areas are not suitable for constructed wetlands8.
Previously described techniques are cost-effective methods to dispose of wastewater in remote areas with limited access to centralized sewer systems. However, these methods often involve minimal or no treatment and may pose environmental risks. In developed areas prone to flooding, where property owners cannot connect to a centralized sewer system, zero liquid discharge (ZLD) based onsite wastewater treatment plants are a more sustainable alternative.
6. Onsite Zero liquid discharge (ZLD) wastewater treatment system
ZLD system treats wastewater to a high standard, enabling reuse and eliminating liquid effluent. While ZLD systems are more expensive, they provide a comprehensive solution for areas with strict environmental regulations or limited water resources. The components of an onsite ZLD wastewater treatment plant are as follows.
- Primary treatment: The primary treatment is carried out by an underground septic tank where anaerobic digestion reduces the BOD load by 25-45%9. After the required detention period, the wastewater is pumped to the subsequent unit.
- Secondary Treatment Unit: This unit introduces air into the wastewater, allowing aerobic bacteria to break down organic pollutants i.e. BOD effectively. A variety of techniques can be used as secondary treatment i.e. activated sludge process (ASP)10, trickling filters, sequencing batch reactor (SBR) etc. Commercially available systems such as the Nayadic wastewater treatment unit11 can also be employed for this purpose. Secondary treatment plays a crucial role in significantly reducing the pollution i.e. BOD and TSS load is reduced from 60-80%12 making the wastewater safer for further treatment or discharge.
- Tertiary Treatment: In this stage, the wastewater is further polished using advanced methods such as sand filtration and reverse osmosis (RO) units to reduce total dissolved solids (TDS), total suspended solids (TSS), and pathogens. The RO process produces permeate, which can be reused, while the concentrate can be directed to a constructed wetland. In the wetland, plants can absorb nutrients from the concentrate, or the water can evaporate, leaving behind dry waste that is easier to manage and dispose of.
Onsite Zero liquid discharge (ZLD) wastewater treatment system offers a comprehensive solution for treating wastewater to potable standards, enabling its reuse. This system including all necessary equipment and containers, can be installed in a medium-sized space, either underground or above ground. ZLD systems are adaptable to various locations, regardless of land issues, soil type, groundwater table depth or flood hazards. While ZLD systems can be more expensive, their benefits, such as replenishing a vital resource “water”, often justify the cost.
The choice of wastewater disposal/treatment method depends on factors such as population size, geographic location, climate, and available resources. It is important to select appropriate methods that are both effective and sustainable for the specific needs of the community.
- Deshmukh, M. S. (2013). Conditions of slum population of major sub-urban wards of Mumbai in Maharashtra. Voice of research, 2(2), 34-40. ↩︎
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- https://repository.lboro.ac.uk/articles/conference_contribution/A_new_innovation_for_lining_pit_latrines_in_collapsing_formations/9593768?file=17233928 ↩︎
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