Standardisation of Design and Maintenance of DEWATS Plants in India
S.Ramesh Sakthivel*, A.Seshadri**, Md.Azizu Rahman*, V.M.Chariar*
* Centre for Rural Development and Technology, Indian Institute of Technology, New Delhi-110 016
(E-mail: [email protected]; [email protected]; [email protected])
** The Vigyan Vijay Foundation, H-2/2-5, Mahavir Enclave, PalamDabri Road, New Delhi-110 045
(E-mail: [email protected])
Abstract
In India as well as in several developing nations, the issue of sewage treatment and disposal is a big
challenge. High capacities as well as heavy financial investments are required to address these
issues. Promoting decentralised waste water treatment systems both at individual and institutional
levels can partially help in addressing this challenge. Considering these aspects, The Vigyan Vijay
Foundation (VVF), New Delhi implemented construction of over 14 DEWATS plants in Northern
India with the active collaboration of Centre for Rural Development and Technology, Indian
Institute of Technology Delhi (CRDT, IIT Delhi). Based on the experience of promoting DEWATS
plants, the design of DEWATS plants was standardised for enabling engineers to rapidly implement
rapid design of the plants. This paper summarises the key design parameters of DEWATS plants
developed, average cost of construction of 1 KLD of DEWATS plant and also lists maintenance
aspects to be taken care of in the DEWATS plants.
Keywords
DEWATS; design; economics; maintenance; wastewater
INTRODUCTION
Sewage disposal and treatment is a major concern in India and in several developing nations (Singh et
al., 2011). In class I and II cities of India, sewage treatment capacity of only 6,190 million litres per
day (MLD) has been installed against the total sewage generation of 29,129 MLD (Nadeem et al.,
2008). The untreated sewage finally ends up in water bodies located near the cities and towns. In the
guidelines for water quality released in India, 82 locations were classified as sensitive zones prone to
be affected by serious water quality contamination in some of the major water bodies of the country
(CPCB, 2008). Untreated sewage flowing into the water bodies from various towns and cities has been
attributed as the main cause of water contamination. Release of untreated sewage in these water bodies
can cause health hazards which seriously affect the well being of children as well as people belonging
to economically weaker sections. Diarrhoea is a condition caused by consumption of contaminated
water leading to gastrointestinal infections which kill around 2.2 million people globally each year, the
primary victims of waterborne diseases are mostly children in developing countries (WHO, 2000).
Another deleterious impact of untreated sewage in waterbodies is adverse impact on aquatic life due to
eutrophication.
Promotion of decentralised wastewater treatment systems (DEWATS) both at individual and
institutional levels can offset this problem to a greater extent. DEWATS technology reduces the
dependence on centralised treatment facilities. Bio-remediation of waste water is an efficient process
and it also enables recovery of large volume of wastewater for useful purposes especially in urban
areas for sustainable use of water. Since 2000, The Vigyan Vijay Foundation (VVF), New Delhi has
constructed over 14 DEWATS plants in Northern India with the collaboration of Indian Institute of
Technology Delhi. The foundation has also actively promoted the DEWATS concept in India through
a number of capacity building programmes and advocacy initiatives. In 2009, the effort of VVF was
recognised by BORDA – CDD and it was selected as a member of the network for the promotion of
DEWATS plants in India. This paper outlines the standard design procedure, economics and
maintenance aspects based on the experience of VVF and IIT Delhi in promoting DEWATS plants in
India.
MATERIALS AND METHODS
Based on the experiences of promoting DEWATS plants in India (Table 1), simple standard design
procedures have been developed for rapid design of DEWATS plants. These steps have been found
useful by environmental engineers and other professionals who have been trained on DEWATS
methodology and technology. The design primarily utilises the quantity of sewage flow calculated in
litres/m3 per-day and the Hydraulic Retention Time (HRT) chosen for a location. Using the total
volume of waste water to be handled based on these two parameters, the design capacity of various
elements of a DEWATS plant such as anaerobic settling tank, anaerobic septic tank, anaerobic baffled
filter- reactor and planted bed filters are worked out.
Table 1. DEWATS plants constructed in different locations and their performance
Location
1
2
3
4
5
Centre for Science and
Environment, Delhi
Indian Institute of
Technology Delhi
Vasant Vihar Park,
New Delhi
Ashram, Utharakand
Scindia School,
Gwalior
BOD
Plant
Capacity
(KLD)
Year
Inlet
Outlet
Efficiency of Plant
( in % of reduction
in BOD)
Sewage
8
2002
300
20
93
Drain Water
10
2002
200
20
90
Drain Water
40
2002
350
30
91
Sewage
30
2004
270
25
91
Sewage
15
2006
300
20
93
Type of
Influent
In addition, information on the average cost per kilo-litres per day (KLD) of medium sized DEWATS
plant can help in rapid estimation of the project cost for a proposed location. For this purpose, the
actual cost incurred for the construction of DEWATS plants at various locations were recalculated
based on present construction costs. For this purpose, 5 DEWATS plants implemented by VVF and
IIT Delhi were selected.
Periodic maintenance of DEWATS plants is crucial for ensuring their successful operation. Apart from
some standard procedures which have been identified by agencies working on this technology, some
innovative methods have been identified for easing maintenance routines and increasing the
performance of DEWATS plants. These methods have been proposed based on the experience of VVF
and IIT Delhi in undertaking maintenance works of plants constructed in India.
Figure 1. DEWATS plant constructed at Centre for Science and Environment, New Delhi
RESULTS AND DISCUSSION
Standard Design
Although the components of DEWATS can be designed independently based on the expected inflow,
strength of sewage and other design parameters such as HRT (Sasse, 2008), this becomes an elaborate
procedure for an environmental engineer who is new to the concept of DEWATS plants. Therefore, a
rapid designing procedure was developed. This method involves arriving at the total capacity (volume)
of DEWATS plant needed based on the daily sewage flow and HRT (Table 2). Finally, the estimated
total capacity arrived at can be proportionately assigned to various components of a DEWATS plant.
In general, for DEWATS plants implemented in most parts of India, fixing a volume 30% of the total
estimated capacity individually to anaerobic septic tank, baffled reactor, planted bed filter, which are
the important components of a DEWATS plant, was found to be adequate. The other secondary
components like in the DEWATS plants such as settling tanks and storage/polishing tanks can be
allocated the remaining volume of the estimated capacity or as desired. Other design parameters such
as hydraulic slope (1%), pipe dimensions (minimum 100 mm) and free board for all tanks (0.300.45m) have to be adopted.
Table 2. General design parameters suggested for design of a DEWATS Plant
Description
HRT
Total Capacity of Plant (V)
Inlet, outlet and other pipes
Free Board
Manholes
Ideal Design Capacity of a Single Unit
Suggested Design Parameters
8-10 days (up to 25 days in cold regions)
Flow per day in KLD x HRT – No. of Days
1% slope between outlet and inlets. Minimum pipe
size 100 mm dia provided in staggered manner
0.30 m to 0.45 m above the suggested depth
Minimum 450 mm wide/dia above each separated area
of tanks
Upto 25 KLD
Pre-treatment (Anaerobic Settling Tank). The pre-treatment of waste water entering the plants is
carried out using an anaerobic settling tank/chamber located at the beginning of the DEWATS module.
In the proposed design, allocation of 5% of the total volume of DEWATS plant is suggested for this
purpose (Table 3). A shallow tank having 1m effective depth allows settling of suspended particles. A
baffle provided with an opening at the bottom facilitates effective solid-liquid separation. The tank
with a minimum width of 1.2m is required for easy operation and maintenance process. Length of the
tank is worked out based on the depth and breadth of the plant adopted. A free board of 0.30 to 0.45 m
must be provided in addition to depth of the tank chosen.
Table 3. Design parameters for pre-treatment in the anaerobic settling tank
Description
Suggested Design Parameters
Capacity (V1)
5% of Total Capacity of Plant
Depth (D)
1.2 m + Free Board 0.3-0.45m
Breadth (B)
Minimum 1.2m (for O&M)
Length (L)
Baffles
V1/(DxB)
1 no.(Volume of tanks - 50:50)
Opening between baffles
Bottom of the baffle
Primary Treatment (Anaerobic Septic Tank). The primary treatment of waste water is effected in an
anaerobic septic tank for anaerobic degradation of suspended and dissolved solids. A volume of 30%
of the total design volume of the DEWATS plant is suggested for the anaerobic septic tank. The
volume can be reduced up to 15% of the total design volume if the proportion of grey water in the
inflow is very large. A standard depth of 3m and a minimum breadth of 1.2m are suggested (Table 4).
The dimensions suggested can be altered to suit the site conditions. The tank is partitioned into two
portions with a baffle constructed at the centre. An opening provided around the middle level of the
tank connects both the units. In order to allow passage of gas generated due to digestion of solids, a
common vent pipe of minimum 100m dia with a cowl on top should be firmly fixed over the tank slab.
Small openings of 100mm dia. are provided at the top of the baffle to allow gas to flow through the
single gas vent pipe provided.
Table 4. Design parameters for primary treatment (anaerobic septic tank)
Description
Suggested Design Parameters
Capacity (V2)
30% of Total Capacity of Plant
Depth
3m
Breadth (B)
Minimum 1.2m (for O&M)
Length (L)
V2/(DxB)
Baffles
1 no. (Volume of tanks - 50:50)
Opening between baffles
At middle of the baffle
Secondary Treatment (Anaerobic Baffled Filter Reactor). The secondary treatment module involves
using an anaerobic baffled reactor for the treatment of non-settleable and dissolved solids by bringing
them in close contact with a surplus of active bacterial mass. Graded gravel is used as filter media for
the purpose of filtration and to enhance the bacterial action (activated sludge decomposition). A tank
volume of 30% with 3 to 4 baffled separations is suggested for secondary treatment tanks. The volume
of tank is further increased due to space occupied by the filter media which usually has a pore space of
40% for a height of 2m (Table 5). If accurate calculation of pore space is desired, it can be calculated
to by stocking the filter media to be used in a small container of known volume and pouring measured
quantity of water.
Table 5. Design parameters for secondary treatment (anaerobic baffled-filter reactor)
Description
Capacity (V3)
Depth
Breadth (B)
Length (L)
Baffles
Connection between
baffles
Placement of Filter
media
Filter media
provided in Tanks
Suggested Design
Parameters
30% of Total Capacity of
Plant
3m
Minimum 1.2m (for O&M)
((V3+(V3x0.4))/(DxB))
3-4 nos. (equally spaced)
PVC pipes positioned in
baffles carry influent below
the filter media
2m thick placed above the
bottom level of tank
Stones 20-30 cm size and
gravel (40mm and 20mm)
Normally, filter media in the anaerobic baffled reactor is usually placed over raised platforms with
perforations above floor level of tanks to enhance settling of solids at the bottom of the tank. However,
this procedure is expensive and also poses difficulty during maintenance operations. In the suggested
design, filter media is placed in a graded manner right from the bottom of the tank for reducing the
cost and also reduce difficulties in maintenance process. Large stones can be placed at the bottom for
allowing settling of solids in the gaps created. Use of 2 or more PVC pipes of 100mm dia to connect
the baffled tanks placed in a staggered manner is suggested. These pipes have to be firmly fixed with
the baffles during construction stage of the plant. The pipes allow flow of influent waste water beneath
the filter media for effective contact with the filter media.
Table 6. Design parameters for tertiary treatment (planted bed filter)
Description
Capacity (V4)
Depth
Suggested Design Parameters
30% of Total Capacity of Plant
1.2 m
Breadth (B)
Minimum 1.2m (for O&M)
Length (L)
(V4+V4X0.6)/(DxB))
Graded gravel (equal layers of 40
mm and 20mm thickness)
Mud/Soil Balls of 20cm thick
placed between the gravel layer .
Each Ball to contain a plant- Rootzone process
Canna Indiaca, Typa, Reeds 300x300mm spacing
Filter media
Plant Bed
Type of Plants
Tertiary Treatment (Planted Bed Filter). The tertiary treatment unit of a DEWATS plant is a planted
bed filter. A volume of 30% should be allocated to planted bed filter which provides root zone
treatment. The volume of planted bed filter is further adjusted to account for the loss of space occupied
by the filter media (Table 6). The percentage of volume lost due to filter media is normally taken as
60% of the total volume. In the design, planted bed filter of 1.2m depth and free board of 0.30-0.45m
and a minimum width of 1.2m is suggested. The open tank bed is covered with graded gravel of 2040mm and soil balls with a plant stem at each ball at top spaced as per requirement of plant variety
chosen. A balancing tank is provided at the end of the planted bed to regulate water level in the
planted bed.
Final Treatment (Polishing and Storage).
The final treatment of DEWATS plant consists of storage tank and or an open polishing pond. A
minimum volume of 5% is suggested. However, the size of tank can be further increased based on the
volume of water required to be stored in a given location for the purpose of reuse. Use of polishing
ponds with water fountains, aquaculture and water based plants could further improve the quality of
water and also provide an aesthetically pleasing look around DEWATS plants. Constructed area over
the DEWATS plants could be covered with grass and ornamental plants leaving the areas provided for
access manholes. This helps in providing aesthetically pleasing look and also aids in absorption of any
foul gases emanating from DEWATS plants.
Validation of the Proposed Design Procedure
The effective performance of DEWATS plants constructed by VVF based on this procedure is a proof
for validity of the proposed simplified design procedure (Table 1). In addition, inferences gathered
from the database of projects implemented by CDD in India were also utilised to validate the proposed
design procedure. In this analysis, the design of key components (septic tank, anaerobic baffled filter
reactor and planted bed filter) of project sites were calculated based on the proposed design steps and
it was compared with the design adopted by the CDD in these sites. The daily inflow of waste water in
the sites considered for this analysis varied from 1.5 to 307 m3 per day. In this analysis, it was
observed that the total capacity of DEWATS plants adopted in the case studies are quite comparable to
the values obtained using the proposed design procedure for the given inflow of waste water in these
sites. However, the capacity of septic tank worked out using the proposed method varied significantly
in some cases where large proportion of grey water was part of the waste water inflow. Therefore, the
capacity of septic tank could be proportionately reduced if large volume of grey water is expected in
the inflow.
Economics
The data on cost per KLD of DEWATS plants can assist planners and engineers to arrive at a decision
quickly. In this regard, based on the 5 selected DEWATS plants implemented by VVF, the average
cost per KLD of DEWATS has been worked out (Table 7). The cost per KLD of a standard DEWATS
plant in India works out to Rs.66,000/= per KLD (standard deviation Rs.4,183/=) with the current
price of construction materials and labour. This estimated average cost per KLD is expected to hold
good for medium sized DEWATS plants with capacities up to 50 KLD. The cost per KLD worked out
is also in agreement with most of the project datasheets prepared by the Consortium for DEWATS
Dissemination (CDD) Society. However, the cost per KLD for small DEWATS plants implemented
was found to be quite high. The cost per KLD for a DEWATS plant worked out for individual house
with 0.30 KLD was Rs.1,50,000/ per KLD. Sasse et al. (1996) in an analysis found that the cost per
cubic meter capacity of DEWATS plant of small and large sized DEWATS plants was higher than
medium sized plants. The increase in cost was attributed to the number of modules to be included for
small capacity systems and also the sophisticated treatment required for large treatment plants.
Table 7. Average cost of 1 KLD capacity DEWATS plant
1
Centre for Science and
Environment, Delhi
8
2002
2,50,000
Estimated
cost per
KLD as
on date
(INRs.)
5,60,000
70,000
2
Indian Institute of
Technology Delhi
10
2002
3.50,000
7,00,000
70,000
3
Vasant Vihar Park, New
Delhi
40
2002
8,00,000
24,00,000
60,000
4
Ashram, Utharakand
30
2004
7,50,000
19,50,000
65,000
5
Scindia School, Gwalior
15
2006
3,50,000
9,75,000
Average
Std. Deviation
65,000
66,000
4,183
Location
Projected
cost as on
date
(INRs.)
Plant
Original cost of
Capacity Year
Construction
(INRs.)
(KLD)
Maintenance of DEWATS Plants
Maintenance of DEWATS plants is a challenge as it is a cost and labour intensive process. The
maintenance routine of DEWATS involves checking of flow, removal of grease, maintenance of water
level and weeding of planted bed filter (CDD, 2010). In addition to these basic maintenance aspects,
effective maintenance of filter media and planted bed filters can enhance performance of the
DEWATS plants. These steps effectively reduce the frequency of maintenance and there by require
minimum amount of waste handling. In addition, involving people who are already trained in waste
water handling could be helpful in the maintenance of DEWATS plants due to the prevailing cultural
sensitivity towards handling waste in India. Training people skilled in gardening (traditional malis)
also can help in effective utilisation of recycled waste water.
Care of Filter Media: VVF has been supporting owners of the DEWATS in the actual maintenance
works. During the maintenance works, it was found that the removal of filter media from the anaerobic
baffled reactor is often a challenging task. Finding labour force willing to enter the tanks and remove
the filter media is often difficult. From this experience, VVF has been trying to introduce methods
such as providing filter media in modules placed in a netted cage (plastic) for easy removal. These
modules can be easily pulled out with the help of iron rods with hooks on one end from the tanks
through manhole openings in the cover slab of the tanks. Size of the modules can be as small as
20x20x30cm for ease of handling.
In addition, to prolong the life of filter media, special de-sludging pipes running below the filter media
with removable cap on top can be provided in each compartment of the anaerobic baffled reactor tank
(Figure 2). Smaller suction pipes can be inserted through the pipes placed and this can be connected to
a pump to facilitate periodic removal of sludge accumulating at the bottom of the tank and in the filter
media. The floors can be sloped towards these de-sludging pipes with small sunken areas below them
for the effective removal of solids. Such operations would ensure increased life of filter media without
being removed periodically. Water jetting over the filter media and suction of solids accumulated at an
interval of 6 months would increase the life and efficiency of DEWATS plants.
Figure 2. Arrangement showing de-sludging pipes and filter media placed in modules of netted cage
Planted Bed Filters. Maintenance of planted bed filter is crucial for ensuring effective treatment of the
DEWATS plants. It has been reported that performance of planted bed filters are affected due to
variations in climatic conditions and the best performance is observed in warmer regions (U.S. EPA,
2000). Although most regions in India remain warm in most part of the year, a peculiar problem in the
planted bed filters was observed especially in urban areas. Leaf areas of plants in the planted bed
filters are covered with thick layers of dust due to heavy dust that flows along the wind in urban areas
with lot of construction activities. Therefore, it is essential to remove these dust particles periodically
to ensure proper growth of plants in such locations. Occasional watering over the plants can results in
removal of dust particles accumulated for their effective growth and to increase the performance of
DEWATS plants.
CONCLUSIONS
DEWATS plants can be effectively promoted for treatment of domestic wastewater both in large
settlements and individual households in India. In addition, remediation of waste water flowing in
urban drains using DEWATS plants and utilising the treated wastewater for watering gardens and
plantations in urban areas was found promising. The standard design procedure proposed based on the
experiences would be useful for rapid design of DEWATS plants by engineers with minimum training.
Cost of DEWATS plant per KLD of inflow and the maintenance aspects suggested could also be used
for effective planning and maintenance of DEWATS plants.
REFERENCES

CDD (2010) Operational tasks for the upkeep of decentralised wastewater treatment system
(DEWATS). Consortium for DEWATS Dissemination (CDD) Society, Bangalore, India.

CDD. Project Database of India. Consortium for DEWATS Dissemination (CDD) Society,
Bangalore,
India.
http://cddindia.org/decentralised-wastewater-treatment-systemsdewats/projectdatabase/e/1/1.html (accessed on 2/08/2012).

CPCB (2008). Guidelines for water quality management. Central Pollution Control Board, New
Delhi.

Nadeem, K. Rajiv, S. Raghav, A.K. Mittal, A.K. (2008) UASB technology for sewage treatment
in India, Twelfth International Water Technology Conference (IWTC12), Alexandria, Egypt.

Sasse, L. (2008) Decentralized waste water treatment facility in developing countries
(DEWATS). Bremen overseas research & Development association (BORDA), Germany.

Sasse, L. and Otterpohl, R. (1996) Status report on decentralised low maintenance wastewater
treatment systems (LOMWATS), produced by BORDA in cooperation with AFPRO, CEEIC,
GERES, HRIEE and SIITRAT. Commission of the European Union – Brussels and State Office
for Development Co-operation – Bremen.

Singh, S. Raju, N. J. and Sagar, G. (2011) Process design for decentralized sewage treatment
system with total natural resource management. Int. J. Water Res. Environ. Eng. community
3(11), 233-237.

U.S. EPA (2000). Constructed wetlands treatment of municipal wastewaters manual. Office of
Research and Development, EPA-625-R-99-010, September 2000.

WHO (2000). Global Water Supply and Sanitation Assessment. World Health Organization.
Geneva.