Anaerobic System for Domestic Wastewater Treatment

In decentralised treatment systems various combinations of anaerobic and aerobic treatment modules are possible and widely used for the treatment of domestic (and organic industrial) wastewater (BORDA 2017) like illustrated in Fig. 1 below.

Fig 1: Overview on possible treatment modules used for decentralised wastewater treatment. Source: BORDA (2017)

This factsheet focuses on the combination of the following modules (adapted from BORDA 2017 and TILLEY et al. 2014):

The Anaerobic Settler (AS) is a simple but robust and efficient technology for the primary treatment of wastewater. It consists of a least two, sometimes three chambers and is designed for the removal of suspended solids by sedimentation. The low flow velocity in the AS allows settleable particles to sink to the bottom, while constituents lighter than water float to the surface. Settlers are typically designed for a Hydraulic Retention Time (HRT) of 1.5 to 2.5 hours and achieve BOD (Biological Oxygen Demand) removal rates of 20 to 40%. For more information, see factsheet on Septic Tanks.

The Anaerobic Baffled Reactor (ABR) is a cost-effective and efficient secondary wastewater treatment system consists of a series of chambers or compartments, separated by baffles or walls in which the wastewater flows upstream. The extended contact time with the active biomass in the sludge blankets results in improved treatment and up to 90% reduction in BOD and 40 to 75% reduction in pathogens. Typical inflows range from 2 to 200 m3 per day. Critical design parameters include HRT between 48 and 72 hours, wastewater upflow velocity below 0.6 m/h and number of upflow chambers (3 to 6). You will find more information here.

An Anaerobic Filter (AF) is a fixed-bed biological reactor with one or more filtration chambers in series. As wastewater flows through the filter, particles are retained, and organic matter is degraded by the active biomass that is attached to the surface of the filter material. An HRT of 0.5 to 1.5 days is typical and recommended. A maximum surface-loading (i.e. flow per area) rate of 2.8 m/d has proven to be suitable. You will find more information here.

Advanced anaerobic treatment systems consisting of AS, ABR and AF can be used to treat different types of wastewater, such as domestic, organic industrial, and agricultural wastewater. In general, the three treatment modules are characterised by their simple and robust design. They can be constructed on-site and prefabricated from a variety of locally available construction materials such as bricks, cement hollow blocks, reinforced concrete, fibreglass, PVC (polyvinyl chloride) or plastic, ensuring relatively low investment costs. The non-requirement of electricity and absence of electronic components - as there is no need for aeration and provided the terrain allows to operate the modules in gravity flow mode - allow for low operational and maintenance costs (no moving parts that need to be maintained/replaced regularly). The technologies are suitable for areas where land may be limited since the tanks are most commonly installed underground and require a moderate area.

ABRs and AFs can be installed in every type of climate, although the efficiency is lower in colder climates. Other limitations are that they require expert design and construction and that effluent and sludge require further treatment and/or appropriate discharge. Since bleach and other harsh chemicals kill the bacteria present in anaerobic wastewater treatment plants, the use of such substances must be avoided.

Since the reduction of pathogens is rather low, the effluent of the AF can be polished in a low HRT Horizontal Flow Constructed Wetland (HFCW) and UV-unit for pathogen removal to make it suitable for agricultural use.

The beauty of AS, ABR and AF as part of Decentralised Treatment Systems (DTS) is their simplicity, they work without chemicals or electro-chemical equipment/energy use and have low maintenance requirements. Necessary maintenance activities can be carried out by service providers or by supervised and trained maintenance personnel on-site (adapted from TILLEY et al. 2014, BORDA 2017 and SWW n.y.). The most common O&M tasks are:

• Checking the free flow of wastewater from the source to the primary treatment unit and within inter-module pipes.
• Checking and ensuring that all manhole covers are in place.
• Removal of oil and fats from grease traps
• Removal of scum from Anaerobic Settler
• Desludging of Anaerobic Settler chambers and Anaerobic Baffled Reactor and Anaerobic Filter compartments
• Testing of the treated wastewater

In India, advanced decentralised anaerobic treatment systems consisting of AS, ABR and AF for the treatment of domestic, organic industrial and agricultural wastewater are commonly installed in residential settlements such as Aishwarya Empire Township in Raipur (CDD 2020b), schools, colleges and universities such as the new campus of the Indian Institute of Technology, in Gandhinagar, Gujarat, (CDD 2020a), institutions, factories, hospitals such as Aravind Eye Hospital (CDD 2019) and temples. According to the Consortium for DEWATS Dissemination Society (CDD 2021) more than 400 systems have been implemented across India as of beginning 2021.

Depending on the daily volume and characteristics of wastewater to be treated, the specific area for DEWATS with Planted Gravel Filters is between ca. 4 - 7 m2/m3 per day (1 m3 = 1 kilolitre = 1,000 litres) (CDD 2021), as indicated in Fig. 2 below.

Fig. 2: DEWATS area variation with daily inflow. Source: CDD (2021)

In 2021, the costs for DEWATS with Planted Gravel Filters was between ca. Indian Rupee 40,000 to 110,000 (1.1 lakh) per kilolitres per day (CDD 2021), as illustrated in Fig. 3 below.

Fig. 3: DEWATS cost variation with daily inflow. Source: CDD (2021)

Worldwide, DEWATS systems are increasingly being recognized as a proven and effective solution for decentralized and sustainable wastewater treatment and management, particularly in areas with limited resources and infrastructure.

According to CDD (2021), DEWATS is implemented in over 17 countries. The Basic Needs Services (BNS) Philippines, its partner networks together with the Bremen Overseas Research and Development Association (BORDA), achieved the sustainable operation of more than 900 DEWATS plants worldwide (BNS n.y.).