Bio-Digesters: An Executive Appraisal

Review of recently popular bio-digester systems used instead of conventional septic tanks

Introduction

This article presents an executive appraisal of bio-digester systems used in place of conventional septic tanks, for domestic sewage processing. A system that is gaining popularity is developed and promoted jointly by the Defence Research & Development Organization (DRDO) and Indian Railways. This system works through a process called inoculation – wherein a mixture of different species of bacteria, predominantly inoculum genera, is seeded in the sewage collection tank with predetermined quantity of earth and cow-dung. It is claimed that the bacteria are capable of breaking down sludge into predominantly methane, carbon dioxide and water, through a sequence of microbial digestion processes. The gaseous by-products are vented from the tank and treated sewage effluent (TSE) water is tapped from the water for irrigation purposes.

The sewage collection tanks may be underground or on-ground, and may be constructed using reinforced concrete, masonry or FRP material. The oxygen demand and total dissolved solids content of TSE water can further be reduced by providing a filtration system on ground using a reed bed, to meet the quality standards to be directly let into stormwater drains or water bodies. An indication of the arrangement of the bio-digestor, with the advantages of the system is presented in the following figure.

Design Principles

Schematic arrangement of the bio digestor is shown in the following figure:

 

Conventional septic tanks are designed for holding following volume (excluding freeboard):

1. Sedimentation: About 0.3m depth is provided for sedimentation
2. Sludge Digestion: Per capita volume of sludge digestion is taken as 32L
3. Sludge Storage: Per capita volume if sludge storage is taken as 0.2L per day. For 1-year cleaning, this comes to about 80.3L per capita, including 10% seed sludge.

However, recommendation on Design Specifications is considered based on 14 days holding of sewage volume (@ 80% of 5 LPCD) as 56L per capita. [For Phase 1, this was considered for 2 days holding]

Recommendation from Indian Railways for bio-digester proportioning is 45 LPCD x 72 hours retention = 135 litres per capita (excluding freeboard).

A comparison of effluent water characteristics, as claimed by DRDO is presented as follows:

Limitations of Bio Digestor Systems

1. Bio digestor is subject to a set of Dos and Donts to ensure proper functioning. An important limitation is that non-bio-degradable materials such as plastics, glass etc shall not be disposed into the system, as the same results in choking of digestor.
2. A 4-year study on stationary bio digest tanks reported in 2017 by IIT Chennai refutes that organic matter does not undergo any kind of treatment in bio-digesters superior to conventional septic tank. Ministry of Defence has, however, questioned the experiment design of IIT Madras and this apprehension has not since been resolved.
3. The output water quality Standards need to assessed against the standards of Pollution Control Board, to obtain permissions to let water into groundwater or streams.
4. DRDO system is neither patented nor is the chemical equations of organic matter breakdown clearly specified. The design of tank for the bio-chemical processes are based on the thumb-rule style recommendations of DRDO.