MODEL MATCHING TREATED EFFLUENT QUALITY TO NON –
POTABLE WATER REUSES IN SOUTH AFRICA
J.R. Adewumia, A.A. Ilemobadea and J.E. van Zylb
a
School of Civil and Environmental Engineering, University of the Witwatersrand, Johannesburg. South Africa
b
Department of Civil Engineering Science, University of Johannesburg, South Africa
ABSTRACT:
Growing environmental awareness and increasing freshwater resources scarcity has given rise
to the use of treated effluent as an unconventional source of water supply in many arid and semiarid regions of the world in which South Africa can be classified. To ensure environmental
sustainability, food security and to promote economic growth, the society will be compelled to
adopt treated effluent reuse strategies. However, the amount of treated effluent that can be
reclaimed for reuse is highly dependent on a number of factors, ranging from technical to socioeconomic and institutional conditions. Technically, if the treated effluent quality from a treatment
works is to meet all potential end uses, it is necessary that the quality be satisfactory. If however,
several units are involved in the treatment processes, with each unit producing different effluent
qualities, it is necessary that scenarios be built to appropriately match the qualities required for
different potential end uses and the treated effluent quality produced. This is necessary to
determine if further treatment is needed to meet users’ requirements and to protect public health.
Planning this is a very complex exercise, which needs to consider the selection of additional
treatment processes required to achieve the desired effluent quality and the selection of possible
end-users with differing quantity and quality requirements. In any case, the use of adequate tools
to build scenarios is paramount. Facilities ranging from Decision Support Systems to the
simplest analytical tools could help the stakeholders to select the best match between the user’s
quality requirements and the effluent quality and take the final decision whether the scheme can
be implemented or not. This paper provides a systematic framework for the analysis of potential
treated effluent reuse under various driving forces and constraints. The input data for this model
includes information on water quality requirements for different types of end users of treated
effluent, design and costing of additional treatment unit process, suggestions for treatment matrix
that could be used and the rules for combining unit processes. This model will guide the
responsible authority to take the most economical reuse option in any locality based on the
existing wastewater treatment infrastructure.
Keywords: Potential uses; Quality requirements; Support tool; Wastewater reuse
Sub-theme: Water and Environment


Corresponding author: E-mail address: James.Adewumi@students.wits.ac.za (J.R.Adewumi)
1
1.0
INTRODUCTION
As both industry and populations continue to increase, there is an increase in wastewater
generation in urban areas and a great decrease in freshwater availability. Community concerns
about environmental pollution resulting from the quality of wastewater disposed to sensitive
environments has led to pressures on water industry to treat wastewater to a higher level before
discharge to receiving rivers or streams. As this trend continues, wastewater reuse is gaining
popularity as an unconventional source of non-potable water in different countries around the
world. While the nutrients in wastewater can assist plant growth when reused for irrigation, their
disposal, in extreme cases, is detrimental to ecosystems of the receiving environment. Also, water
scarcity experienced globally has led to the embracing of wastewater, salinewater and greywater
reuse in many large urban areas in regions previously considered to have sufficient water sources
like China (Junying et al, 2004, Weizhen and Andrew, 2003), Japan (Dixon et al, 1999),
Germany (Nolde, 1999), United Kingdom (UKEA, 2000) and the United States of America
(Okun, 1996).
A large part of South Africa is predominantly located in a semi-arid part of the world with a
mean annual precipitation of about 450 mm, which is well below world average of about 860 mm
per annum. This low quantity of precipitation is highly uneven and also accompanied with high
evaporation rates which make the country’s water resources extremely limited and scarce. Also,
the surface runoff is highly variable and stream flow in South African rivers is at relatively low
levels for most of the time thereby, limiting the proportion of stream flow that can relied upon to
be available for use without adverse effect on the aquatic plants and animals (DWAF, 2004).
The scarcity of water in South Africa is also aggravated by pollution of the surface and
groundwater resources. An indication of the pollution pressure on South African freshwater
resources could be found in Vaal Barrage catchment that supplies freshwater to Gauteng province.
The catchment receives 859 Ml/d of domestic effluents, 240 Ml/d of mine effluents and about
100Ml/d of industrial effluents (NSER, 2007). This results in increase in phosphate, chemical
oxygen demand (COD), ammonium, suspended solids, faecal coliforms, sulphate, metals
(manganese, aluminium, iron) and a decrease in pH of the Vaal River. Also, in the City of Cape
Town, algae blooms and geosmin in the raw water is increasingly becoming problematic,
particularly at Theewaterskloof, Voelvlei, Steenbras and Constantia Nek. (CCT, 2007).
2.0
OVERVIEW OF WASTEWATER TREATMENT DECISION SUPPORT
SYSTEMS
Several contributions have been made in wastewater treatment plant design by the use of
computer programs. Many of these programs simulate treatment trains, evaluate, screen and
select the optimum treatment scheme using different techniques to generate and screen unit
process combinations. Computer technologies have been adopted in solving specific problems in
the field of wastewater treatment by adopting routine computational techniques (Chang and Liaw,
1985; Gasso, et al, 1992). With the growth and confidence in algorithm development and
optimization techniques, more sophisticated problems are solved using various research tools like
Monte Carlo simulation (Chen and Beck, 1997), expert systems (Ahmed et al, 2002;
Economopoulou and Economopoulos, 2003), integer programming (Balkema et al, 2001), stepwise approach with genetic algorithm (Jaksimovic et al, 2006; Jasmovic, et al, 2008) and multi
criteria analysis (Hidalgo, 2007). Most of these models based their evaluation on technical
functionality and economic factors without detailed consideration of other important factors like
environmental and socio-cultural factors while suggestions for complete treatment modules were
2
based on local conditions which limit their practical applications globally. However, Eliss and
Tang (1990) and Tang and Ellis (1994) considered many other factors in their model but it proves
to be too ambiguous to use in other locations because of their non-flexibility. The objective of
this study is to present a model for selecting treatment unit processes that will treat municipal
wastewater to the quality required to meet end uses. This model will use multi-criteria factors in
selecting optimum treatment processes suitable in any local condition.
2.1
Components of a Wastewater Treatment Decision Support System
In general, Decision Support System models have three system components namely: knowledge
base, user interface or control module and reasoning engine.
 Knowledge base
The knowledge base is encoded in the form of case based and rules. The case based reasoning is a
computer based problem solver which uses the solution from an established case to solve existing
problems. A set of rules are formulated and coded to allow the system deduce new results from
the initial set of cases. The robustness of any model highly depends on the information (number
of unit process, cost, etc.) contained in the knowledge base. Most of the models developed to
simulate treatment trains for wastewater reuse have a limited number of unit processes with
limited number of rules that describe the acceptable combinations of unit processes in treatment
trains.
 Reasoning engine
This is the thinking machine of the system dealing with the solving of the problem by interpreting
the knowledge activated by case based and rules. With the help of any suitable advance
computing programming language, the developed algorithm is coded for easy interpretation by
computer.
 User interface
The user interface provides interactive access to the input and output. Modifications can be made
easily and the results immediately updated during the design process. It includes natural
communication medium with the users, reasoning explanation interface, other information
entered by the users and computing results. The information required in the Input unit is the basic
data about the operating systems and constraints and includes the characteristics of wastewater
(BOD, TS, TSS, N, P FC, heavy metals, etc.), land requirements, energy consumption and the
proposed water reuse purposes and quantities. In the output unit, information such as optimum
treatment schemes, the estimated capital and operational costs are displaced for the users.
2.2
Indicators For Selecting Wastewater Treatment
Wastewater treatment process schemes are categorized as a function of the raw wastewater
quality and the disposal methods or intended reuse application. Different indicators have been
used in the past to generate, screen and select treatment trains in wastewater. The most common
ones are based on technical and economic considerations of the unit process. However, other
important factors have been included in recent times. For example, in addition to economic life
cycle cost, Metcalf and Eddy Inc. (2003) outline 23 important factors that should be considered in
the evaluation and selection of wastewater treatment facilities. In Eliss and Tang (1990); Tang
and Ellis (1994); Lundin et al., (1999); Mels et al., (1999) and Balkema et al., (2001), multidisciplinary sets of sustainability indicators that include technical, economic, environmental and
socio-cultural have been used in the selection of treatment trains for domestic and municipal
wastewater. The differences in indicators used by various researchers depend on the methodology
3
used in the selection of the preferred alternatives as well as the goals and scope of the work (i.e.
methods of disposal – reuse or no reuse).
From a practical point of view, the rates at which physical, chemical and biological reactions and
conversions occur are important as they will affect the size of the treatment facilities that must be
provided. The fundamental basis for evaluating the efficiency of technologies used in wastewater
treatment is the materials mass balance principle in which an account of mass is made before and
after reactions and conversions have taken place.
3.0
MODEL TO MATCH WASTEWATER TREATMENT TRAINS TO NONPOTABLE WATER REUSES
To assist the decision makers in selecting suitable wastewater treatment systems, knowledge of
the efficiency of a wide range of treatment trains should be provided in an unambiguous manner
for adaptation and interpretation according to the local situation. The starting point of building
wastewater treatment trains is the definition of potential reuse applications. In the South African
context, municipal wastewater can be reused for industrial (cooling, boiler feed and process
water), domestic (toilet flushing, garden/ lawn irrigation and cleaning), landscape and
recreational irrigation (Urban), construction and Agricultural irrigation purposes. These reuse
options require water of different quality which can be achieved by using specific method of
treatment, depending on the source quality. In all situations, a series of treatment processes is
needed to achieve the required water quality for reuse through a system analysis approach in
which one chooses from a wide variety of unit operations using case based rules with suggestions
on new combinations for comparing alternatives.
When considering treatment for a reuse system, the overriding concern continues to be whether
the quality of the treated effluent is appropriate for the intended use. Higher level uses, such as
toilet flushing, irrigation of public-access lands or vegetables to be consumed without processing,
require a higher level of wastewater treatment and reliability prior to reuse than with lower level
uses, such as irrigation of forage crops and restricted access area irrigation. For instance, in urban
settings, where there is a high potential for human exposure to reclaimed water used for
landscape irrigation, industrial purposes, and toilet flushing, the reclaimed water must be clear,
colourless, and odourless to ensure that it is aesthetically acceptable to the users. To ensure
minimum health risk to the public, it must also be free from pathogens.
As the first step in developing this model, all realistic processes involved in wastewater treatment
schemes in the South African context are described with special attention to their efficiency in
removing constituents to meet the water quality requirements for various reuse applications. A
clear distinction is made between primary, secondary and advanced treatment processes, which
includes both conventional and innovative options as shown in Table 1.
Table 1: Unit process operations included in the model
Treatment
Unit Process
Preliminary
Bar Screen
Comminution
Grit chamber
Coarse screen
Primary
Sedimentation w/o Coagulant
4
Secondary
Tertiary
Disinfection
Sedimentation w/ Coagulant
Anaerobic ponds
Activated sludge + Secondary sedimentation
Trickling filter + Secondary sedimentation
Rotating Biological Contractors + Secondary sedimentation
Membrane bioreactor
Stabilization Pond: Aerobic ponds
Stabilization Pond: Anaerobic ponds
Stabilization Pond: facultative ponds
Biological phosphorus removal
P-precipitation
Constructed Wetland
Denitrification
Maturation pond
Dual media filter
Micro filtration
Ultra filtration
Nano filtration
Reverse osmosis
Soil Aquifer Treatment (SAT)
Activated carbon
Ion exchange
Advanced oxidation process
Electrodialysis
Chemical precipitation
Chlorine gas
Chlorine dioxide
UV radiation
Ozone
3.1
Selection of Treatment Trains
The combination of unit processes shown in Table 1 to form treatment train is not a very simple
design process. Therefore, a selection has to be made among the treatment unit processes to form
standard treatment trains for reuse purposes. The assemblage of the treatment train from
individual unit processes is simulated using a well developed algorithm. The selection model is
developed using MATLAB and provides a user-friendly graphical interface consisting of forms
used in editing the input and output data and the display of results. The model uses a project file
to store the knowledge base and user entered information. The basic data contained in the
knowledge base are water quality requirements for different types of end uses of treated effluent,
design and costing information on unit processes, suggestions for treatment trains that could be
used for influent quality/end use combinations, rules for combining units processes and the
design costing information for the distribution system components. The end users of treated
effluent in South Africa are classified into six different groups as shown in Table 2. The
classification is based on the current treated wastewater reuse applications in the country.
The key steps in the development of the knowledge base is the collection of information on
individual unit processes that could be used to form treatment trains to be evaluated. Such
5
information includes allowable influent quality design criteria, process efficiencies for a series of
pollutants, land and labour requirements, sludge and concentrate production, cost estimates and
preference scoring on the qualitative evaluation criteria. All this information is displayed to the
decision makers in a series of editable forms, which allow them to review all the information and
alter the calculations to suit their conditions.
Table 2: Reuse activities of treated effluent in South Africa
Reuse Type
Description of Reuse
Domestic
Toilet flushing, garden/ lawn irrigation, home air
conditioning systems, car washing and cleaning
Landscape
and
Recreational Open access landscape areas like school fields, parks, golf
Irrigation (Urban)
courses, sport fields, etc
Industrial
Industrial cooling, boiler feed and process water except for
food industries
Construction
Blocks and concrete making and laying, dust suppression,
composting site, etc.
Agricultural Irrigation (unrestricted) Irrigation of raw consumed food crops, fruit trees sprinkler
irrigation, greenhouse crop irrigation, etc.
Agricultural Irrigation (restricted)
Irrigation of pasture for milking or meat animals, fodder,
cereals, fibres, seed crops and other areas where public
access is prohibited.
3.2
Treatment Scheme Performance
Various processes could be adopted for treatment of municipal wastewater. In this model, the
developed wastewater treatment trains will produce effluent of sequentially improved
characteristics suitable for reuse in different applications as shown in Fig 1. A removal efficiency
of individual process modules has been developed based on the performance of local treatment
plants and as reported in the literatures (Ahmed et al. 2002, Metcalf and Eddy (2003) and
AQUAREC, 2006).
6
Preliminary
Primary
Secondary
Restricted Irrigation
Environmental enhancement
Construction
Tertiary
Unrestricted irrigation
Urban & domestic use
Disinfection
Disinfection
Bar screen
Comminution
Grit removal
Low Rate Processes
Stabilization pond: Anaerobic
Stabilization pond: Aerobic
Stabilization pond: facultative
Constructed wetland
Sedimentation
Membrane filtration
Anaerobic pond
High Rate Processes
Rotary biological contactors
Activated sludge
Trickling filter
Membrane Bioreactor
Sec. Sedimentation
Dissolved Solids Removal
Dual media filter
Micro filtration
Ultra filtration
Nano filtration
Reverse osmosis
Soil aquifer treatment
Phosphorus & Nitrogen Removal
Biological phosphorus removal
Constructed wetland
Maturation pond
P-precipitation
Organics & Metals Removal
Advance oxidation process
Chemical precipitation
Activated carbon
Ion exchange
Electrodialysis
Sludge processing
Disinfection
Industrial
Urban & domestic
Disposal
Fig 1: Wastewater treatment options for various reuse
application
Optimization
Module
3.3
As shown in Table 1, the knowledge base included in this model consists of 34 treatment unit
processes. Evaluating all possible combinations of these unit processes will yield large number of
total possible combinations. However, the analysis of wastewater reuse options can be conducted
in places where some treatment of wastewater is already existing using the municipal wastewater
treatment works. In this regard, the treatment works would be considered as the source of the
effluent for reuse. In this case, rules incorporated in this model will restrict the search space.
In general, the quality of effluent from municipal wastewater treatment works can be classified as
either secondary or primary effluent depending on the existing level of treatment. To conduct a
simultaneous search of the least cost design alternatives based on the best selection of end users
combination and other sustainability indicators (technical, economics, environmental and sociocultural), the optimization algorithm used is a genetic algorithm (GA). GA is a search technique
used in computing to find exact or approximate solutions to optimization and search problems.
Combinations and sequences chosen entirely at random are not feasible and practicable in getting
the desired quality output in an economically feasible space. With the given details of the set of
treatment unit, rules must be defined for combining these to make a complete treatment module
7
that will transfer materials across the entire wastewater infrastructure. Fig 2 shows the
computational algorithm adopted in this model.
Start
Wastewater quality
Available treatment technologies
Reuse options
A
Define rules for forming
treatment matrix
No
All options
ranked?
Yes
All possible
treatment schemes
and reuse options
considered?
No
Select the best options
Printout
Yes
Generate all feasible
options
End
Simulate the performance
of each option
No
All feasible
options simulated?
Yes
Criteria and rules for ranking
and grouping options
A
Fig 2: Computational algorithm
4.0
CONCLUSIONS
Planning a wastewater reuse system is a complex exercise which requires the selection of
treatment processes required to achieve the desired effluent quality. To enable decision makers to
choose suitable wastewater treatment systems, there must be a good understanding of the best
combination of treatment units to form a treatment matrix in a sustainable manner. Therefore, a
decision support system is being developed to compare a wide range of alternative treatment
schemes evaluate on technical, economic, environmental and socio-cultural grounds to produce
water of suitable qualities for particular purposes under prevailing conditions and constraints. The
developed tool will allow efficient exploration of design alternatives to be conducted to assist
decision makers in the development of highly efficient wastewater reuse schemes.
8
5.0
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