China's rural wastewater treatment techs

A research report of Sino-Africa wastewater treatment cooperation was released by Beijing Municipal Research Institute of Environmental Protection and Tongji University, to explore the possible wasterwater treatment technology transfer mechanism between China and African countries.

According to the report, some of the current rural wastewater treatment technologies with low energy consumption and decentralized process include:

(1) Septic tank

Septic tank in rural area is generally based on three grids, which is the most important pre-treatment technology of wastewater used in the South currently, especially suitable for faeces and urine. Septic tanks fully make use of the capacity to settle the high proportion of parasite eggs present in faeces and urine. In the sealed condition, faeces undergo anaerobic fermentation, liquefaction, ammonization and biological antagonism. As a result, bacteria and parasite eggs are eliminated and mosquito breeding is controlled. However, it is difficult for the effluent from septic tank to meet discharge standards. In practice, the combination of septic tanks and other general technology, such as constructed wetlands and soil percolation is utilized.

(2) Biogas tank

Biogas tank can be used as primary treatment of wastewater for a single or joint household in the rural area with the characteristics of low investment, good effect, easy management and no energy consumption. Most organic matters can be removed by the anaerobic fermentation and biogas can be produced. Biogas slurry and residue contain a variety of good quality organic nutrients which can be used as fertilizer in agriculture. This practice has environmental and social benefits. However, it is difficult for the effluent from biogas tank to meet discharge standards, and further treatment with aerobic biological treatment unit may be necessary.

(3) Constructed wetland

Rural wetland that is commonly used is the subsurface water flow wetland. It is a constructed pond or trench, with an impermeable layer at the bottom that is filled with substrate to a certain depth and makes use of a mixture of plants and micro-organisms to purify wastewater. The process has the advantage of being simple and cheap to implement, manage and maintain for the South China area where there are many wetlands and insufficient funds for wastewater treatment. Moreover, the population density is concentrated in small villages.

(4) Stabilization ponds

Stabilization pond is applicable to rural areas with lakes, ponds, depressions and unused water for use, mainly through physical aquatic ecosystems, natural or fortified chemical and biological effects to purify wastewater. Stabilization pond has the advantage of simple design, simple construction, and low maintenance; however, the treatment effect is susceptible to temperature. For cold northern regions, in summer it can be acted as a sewage treatment pond, while in winter it can be used as storage pond for holding the sewage instead of discharging into the river. It is low investment if land is not included. This is applicable to the southern part where the temperature is warmer especially in Cambodia and South East Asia.

(5) Land treatment system

Under controlled conditions, the wastewater is dispersed in to the land to be treated through soil - plant system via a process of physical, chemical and biological purification. Land treatment system has the advantage of low investment, good energy efficiency, and low operating costs and is suited to rural areas with inadequate funds and relative abundance of land. Land treatment generally requires sandy soil with good permeability performance, next to a river and with deep groundwater level (> 1.5 m).

For some rural areas with a developed economy and high water quality discharge requirements, centralized wastewater treatment measures can be adopted, such as activated sludge, biofilm and other technologies depending on the quantity of wastewater to be treated.

(1) Anaerobic biofilm reaction tank

Anaerobic biofilm reactor is suitable for the treatment of wastewater being pre-treated by septic tanks, which is a kind of anaerobic biofilm technology reinforced by the filler. Macromolecular organic matter in anaerobic sewage tank is broken down into small organic molecules, which can decrease organic pollution load of the subsequent treatment unit effectively and improve the removal of contaminants. It has the advantage of being a low-cost investment because of its simple design, construction and maintenance costs. The tank can be buried in the ground and covered with plants to beautify the environment. Removal efficiency of COD and SS is around 40% to 60%, thus a follow-up ecological purification process is needed, such as constructed wetland, ecological filter and land percolation etc.

(2) Biological contact oxidation tank

Biological contact oxidation tank can be constructed as single-family, multi-families or village wastewater treatment facilities. Oxygen, wastewater and filler can be contacted fully by aeration. Microorganisms attached to the filler can remove suspended solids, organics, ammonia nitrogen and total nitrogen and other pollutants effectively. It has the advantage of simple construction, small land occupation, easy operation, and good pollutant removal performance.

(3) Oxidation ditch

Oxidation ditch is suitable to treat wastewater with relatively high concentration of pollutants. Treatment capacity should be large and is suitable for village wastewater treatment. Oxidation ditch is operated based on extended aeration commonly to extend the residence time for wastewater and biosolids, and to decrease organic pollution load. The water quality in the effluent is generally good. Passive oxidation ditch and integrated oxidation ditch are commonly selected to treat rural wastewater.

(4) Biofilter

Biofilter is suitable to treat wastewater in small- or medium-sized settlements where there is less available land area, large terrain slope, large fluctuations in water quality and quantity. It is also suitable for areas with high temperature, as higher temperature will increase demand for water distribution. Microorganisms can be attached on the filter which is filled with gravel or plastics and the filler structures form a layer of biofilms. The contaminants adsorbed are transformed by microorganisms when biofilms contact with wastewater.

Biofilter has the advantage of good purifying effect, low investment, low operating costs and stable treatment effect. An eco-filter, which is an environmentally friendly bio-filter technology, is suitable for rural areas with fund shortage and less abundant land. It makes use of biofilms formed with filler and a miniature ecosystem of aquatic organisms to purify wastewater. The filter of particulate matter is primarily completed by filler, while soluble contaminants are removed by biofilm and aquatic organisms in eco-filter. The demonstration project of an earthworm filter in Yixing, Jiangsu shows that construction costs of filter (excluding pipe network construction costs) is 1656.1 RMB/m3, and the total operating cost of the volume of wastewater treated is 0.671 RMB / m3.

(5) Biological floating island

Biological floating island is suitable for rural areas with developed network and warm climate. After being pre-treated with aeration, rural wastewater is discharged to ponds which have biological floating islands made from lightweight materials, such as plastic foam etc on which epiphytic hygrophilous plants are transplanted. The wastewater is purified through the absorption of nitrogen, phosphorus and other nutrients by the plants on the floating island and some economic benefits are obtained through harvesting the by-products. Currently, biological floating islands have been applied in Zhejiang and Jiangsu.

(6) SBR

SBR is suitable for areas of small wastewater quantity, intermittent discharge, higher water quality discharge requirements, such as folk village and areas surrounding lakes and rivers. It is also suitable for areas of water shortage and land tension in North China. SBR is completed in a tank integrated with the function of aeration, sedimentation and discharge, which has the advantage of simple process, investment and area saving; however, automation systems requirement is relatively high.

(7) General aeration tank

General aeration method is suitable for a large amount of wastewater. It can remove organic matter, nitrogen and phosphorus in wastewater; however, it needs more structures and equipment, involve more complex processes and has high operating costs.

(8) Integrated wastewater treatment plant with small scale

Integrated wastewater treatment plant with small scale is mostly buried units, which is suitable for a few dozen households which are relatively concentrated and do not have a centralized collecting pipeline. It includes anaerobic and aerobic biological treatment devices, such as MBR, SBR, and contact oxidation process. Integrated wastewater treatment plant on a small scale has the advantage of low area occupation, strong resistance to shock loading capacity, easy maintenance and management, modular design and flexible operation which can be operated automatically.