5.1.2 Treatment Process Design

Categories: 5.1 General Design Consideration
Time of issue: 2022-04-28 18:34:28
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Keys to selecting, sizing, and siting the system are identifying the desired level of performance and ensuring that the effluent quality at the performance boundaries meets the expected performance requirements (EPA, 2002).

The key factors influencing the effectiveness of a treatment process include:

social-cultural factors
water supply service level
population density
wastewater flows and variation coefficient
soil and groundwater conditions
topography
land availability
affordability and willingness to pay
technical complexity and capacity of the village
energy requirements and availability

Socio-Cultural Factors

The villager’s perceptions of present situation, interest in or susceptibility to change, cultural acceptance of a particular technology, preferred hygiene practices, and their level of participation in the project must be taken into consideration when selecting an appropriate treatment process. Locations of sanitary systems should be away from important cultural resources, such as religious or historic buildings or cemeteries, which is critical to avoid potential conflicts.

It is important to understand if there have been any previous attempts to upgrade sanitation conditions in the village, and if so, the reasons for acceptance or rejection of these previous efforts should be well understood. Understanding the level of hygiene education in the village is also an important factor to consider. Are there any cultural factors that would affect hygiene practices and technology choice? Can the system be maintained by the local village or will support services be required? Only after these fundamental questions have been answered can an effective approach be selected and endorsed by the community.

Water Supply Service Level

A high level of water supplied or consumed by households means a high level of wastewater is produced. For villages without access to a piped water supply system and rely on hand-carried supplies, water-based sanitation systems are not suitable. For villages with in-house plumbing fixtures, water consumption is relatively high. Treatment processes with large capacity should be selected to cope with the wastewater from toilet flushing, washing and shower activities.

Population Density

High population density means large quantities of wastewater and insufficient space for onsite sanitation systems. Normally, onsite systems may be appropriate with population densities less than 25,000 to 30,000 persons per square kilometer. At higher densities, a clustered or community sewer system may be more appropriate.

Wastewater Flows and Varization Coefficient

Treatment capacity is a key parameter for sanitation system design. The water consumption, discharge coefficient, and wastewater collection rate shall be taken into consideration when estimating the wastewater flows. Exact water consumption data for a specific village shall be obtained through detailed investigation if conditions allow. For a small community that produces less than 50 m³/d of wastewater could employ a simple two-stage anaerobic digester system to treat the wastewater. For larger communities that generate over 50 m³/d of wastewater, an enhanced wastewater treatment system should likely be employed that includes both primary and secondary treatment processes.

In China, many rural residents, who were registered as rural population, immigrated to urban areas for working. These immigrants would go back to hometowns during Chinese traditional festival seasons (such as Chinese Lunar New Year) for reunion with family. This will lead to short-term increase of wastewater generated during the festival seasons. In this case, an equalization tank would be considered to buffer the short-term fluctuation of wastewater flow and ensure the normal operation of wastewater treatment facilities.

According to the Technical Specification of Domestic Pollution Control for Town and Village (HJ 574-2010), due to the widely distribution and low density of population in rural areas, the variation coefficient of domestic wastewater flow is high. The maximum hourly variation coefficient can be about 2.0-4.0 and the daily variation coefficient ranges from 1.3-1.6.

Soil and Groundwater Conditions

For subsurface wastewater infiltration systems, soil plays a significant role in removing pollutants and nutrients. The permeability, moisture, depth of saturated zones and texture of soil determines whether the subsurface disposal approach is appropriate. In addition, if a rocky layer exists near the surface, it will be difficult to dig or bury pipes.

Wastewater that infiltrates the soil or comes from leakages can pose risks to the groundwater. The selected treatment process must be compatible with the groundwater table and avoid contamination of groundwater.

Topography

For sewerage systems using gravity force, they need enough gradient to enable a natural flow. This is difficult to implement where the ground is flat.

Land Availability

In many villages, most land is used for agricultural, aquaculture, and housing. To construct a central treatment and disposal system may require that some land be converted from agricultural use to a wastewater management system. If land availability is a significant constraint, a more mechanized and compact wastewater treatment system may be required to minimize the conversion of land.

It is also important to keep in mind that acquiring large land lots for wastewater management may become more difficult for rural villages in the future. Therefore, to secure larger land areas initially and employing a natural treatment system, such as stabilization ponds or constructed wetlands, may be a useful strategy for future system upgrade.

Affordability and Willingness to pay

Investment and operating costs vary widely depending on the system selected. On a simplified level, investment costs for onsite sanitation are low; investment costs for small-piped sewerage systems are medium and those for conventional sewerage systems are high. Affordability and willingness to pay of villagers for the sanitation systems should be fully considered during process design.

Technical Complexity and Capacity of the Village

The sanitation technology selected shall not be complicated and beyond the ability of the village to manage. The simplest solutions that rely on the least amount of mechanized equipment will be the most sustainable and resilient solution for rural villages. Sophisticated wastewater treatment systems that rely on chemicals or sophisticated equipment may be technically and financially difficult for the village to maintain. It is important to introduce solutions that can be integrated to the existing setting and capitalize on the cultural knowledge of the villagers..

Energy Requirements and Availability

The availability and requirements for electrical energy should be considered when selecting any sanitation system. Utilizing passive systems that require minimum external energy sources have been shown to be more reliable options for rural villages. Sanitation systems that require a lot of energy are commonly bypassed when the villages are unwilling to pay the high energy demand for the system.