Optimizing Wastewater Treatment
' Growing volumes of both industrial and municipal wastewater are being discharged to surface waters but the treatment provided frequently is inadequate to protect the desired uses of the receiving waters. With limited resources in terms of both institutional capacity and finance, governments face difficult choices in optimizing their investments in municipal systems and establishing practical requirements for industrial wastewater treatment. This note presents an approach to making coherent decisions on levels of wastewater treatment.'
In many urban situations, both the municipal sewerage system and the extent of industrial wastewater treatment are inadequate. There may be a municipal sewerage network in place but the coverage is usually incomplete and the level of treatment provided is inadequate. Even where reasonable treatment facilities exist, poor maintenance and operation often result in failure to meet design effluent levels.
In such circumstances, management of industrial wastewater discharges is also frequently poor, with uncontrolled discharges of untreated effluent to surface waters (often drainage or stormwater channels) or to the sewer system.
As a result there are high levels of water pollution and it is not uncommon to have streams or water bodies which are almost or completely anaerobic and heavily polluted with organics, pathogens and heavy metals.
There are several objectives which must be addressed in such a situation:
- the collection and removal of domestic and municipal wastewater to protect public health and to improve the immediate environment (particularly important where inadequate disposal is resulting in groundwater pollution);
- the establishment of an effective industrial pollution control system which will reduce the loads and impacts of the industrial discharges;
- provision of municipal and industrial treatment as necessary to protect the environment at the point(s) of final discharge;
- achievement of all of these in an efficient and cost effective manner, within the relevant social and political constraints.
Public and Private Involvement
The basic responsibility for municipal sewage lies with the government (at the appropriate level, preferably local) while industrial wastewater treatment is fundamentally the responsibility of the enterprise but in practical terms has to be driven by government action. The challenge for the government is to use the whole range of options and instruments available to achieve the set of objectives outlined above, combining physical and operational improvements in the municipal infrastructure with the controls and incentives necessary to induce improvements in the industrial sector. This note focuses on the management of industrial wastewater, within this broader context.
Focus on Water Bodies
From the environmental point of view (as opposed to the provision of sanitation services) the focus must be on the receiving waterbodies. The problems are typically diffuse with hundreds or thousands of small discharges and with the problems concentrated to some extent where particularly polluted streams or poorly treated effluents discharge to major water bodies. Upgrading or extension of the wastewater collection system may reduce this diffuse pollution while at the same time resulting in major point discharges which must receive adequate treatment.
A wastewater strategy must therefore be based on a water quality plan for all the receiving waters in the catchment, usually based on water quality objectives.
Water Quality Objectives
It is necessary to have explicit medium to long term objectives for the quality of water in the various waterbodies in the catchment under consideration. These objectives are often based on defined beneficial uses for the waterbodies, typically including about a half dozen such as: water supply source, agricultural use, fisheries protection, etc.
Development of Strategy
Estimation of Loads
The first step in the development of a wastewater strategy is the estimation of the overall loads in the catchment, over the time scale being considered, which is typically about twenty years. As well as information on population growth and densities, this will require estimates of industrial activity and changing patterns.
In some cases, direct observations of industrial pollution loads are available but more often estimates are typically based on statistical information on economic activity (sectors, employment, turnover etc.) using various coefficients for the unit loads of pollution. (A number of different models of pollution loads are available. (See Note forthcoming.)
Overall planning requires estimates of both domestic and industrial loads, on a geographical basis and over the time period under consideration. The estimates need to be developed for key parameters such as suspended solids, oxygen demand, nutrients, organic materials, heavy metals etc., depending on the particular characteristics of the catchment and receiving waters.
Determine Necessary Reductions
Once the load estimates are available, it will be possible to determine the reductions (in present and future loads) that would be necessary to achieve the water quality objectives. In simple cases, a mass balance may suffice but often it will be necessary to carry out water quality modeling (see Note in Water Quality Models).
The objective of the modeling is to estimate the impacts of the increasing loads on water quality and to identify where load reductions are required in order to achieve the water quality objectives. The sophistication required in the modeling will depend on the conditions. In some case a simple one-dimensional model of oxygen depletion will be acceptable: in other cases complex models will have to be developed to address water circulation and the degradation and interaction of several pollutants.
Options for Load Reduction
Once the desired degree of reduction in pollutant loads has been estimated, it is possible to develop options for achieving that reduction. If the most significant pollutants are those associated with industrial effluents, for example, complex organics or heavy metals, then the control efforts will clearly be concentrated on the industrial discharges. However, it is often the case that oxygen depletion and nutrients are the critical issues and the causes are typically a mixture of municipal and industrial. In such cases it is necessary to control both types of sources.
The costs of clean up of a major industrialized urban area can be massive. The estimated costs of water pollution control in Shanghai were $1.4 bn in 1986. Preliminary estimates show the sewerage authority for Buenos Aires facing a nearly $1bn investment program over the next decade. Clearly such programs require decades for implementation and so it is important to tackle them in an organized and cost effective manner.
Components of an Urban Wastewater Program
An urban wastewater program will comprise several distinct but interlocking components. Municipal system improvement will almost always be a central feature but the emphasis given to the industrial wastewater control component will depend very much on the extent of the industrial contribution to the overall problem, the types and sizes of industries involved and the costs of both enforcement and implementation. In some cases (or for some pollutants) small scale or non-point sources may be a significant problem and this is typically difficult to tackle.
Municipal System Upgrading
There are normally two imperatives behind municipal system upgrading:
- expansion of the coverage and quality of sewerage provision;
- and reduction of the impacts of final disposal of treatment plant effluents.
Expansion of the coverage of the service is generally beyond the scope of this note. However, given limited funds, sewerage authorities often have to make trade-offs between expanded coverage and higher levels of treatment, with consequent implications for receiving water quality.
The impacts of final disposal depend, obviously, on the discharge location but in many cases an existing system configuration more or less limits the choice of the discharge site and therefore the emphasis is on improving the level of treatment provided.
Levels of Treatment
Municipal wastewater systems are normally designed to treat influents which are essentially domestic in nature and such systems are ineffective at removing some industrial pollutants and may even be damaged by them.
Design of municipal wastewater treatment is a sophisticated process but in general terms there are three major types of process, in ascending order of removal effectiveness (and cost): physical (sometimes assisted by chemicals); biological; and 'advanced' which includes further chemical and/or biological stages and/or filtration.
These systems can achieve high levels of removal of organic material and of suspended solids and the advanced systems can also remove nutrients to high degree.
Municipal systems do not cope well with high concentrations of complex organic chemicals (such as solvents and hydrocarbons) or of heavy metals. The removal efficiencies are low and biological treatment systems can be poisoned if incoming levels are too high. Other wastewater treatment processes are available which can be tailored to deal with such industrial effluents.
For this reason (and to protect the physical infrastructure and workers) it is normal practice to require pretreatment of industrial effluents which are discharge to a sewer systems.
Industrial Effluent Control
In dealing with industrial effluents, treatment systems can be designed to provide any required level of pollutant removal, although at increasing cost and sometimes with a resultant wastewater treatment sludge which presents its own disposal problems. (Where effluent treatment costs are high, waste minimization programs become very worthwhile.)
The degree of industrial effluent treatment required is established, in theory at least, in relation to relevant ambient quality or effluent standards. In practice, control of industrial effluents is frequently poor and industry may be a major contributor to the overall pollution load.
Where practical controls exist, industry is typically faced with two choices: direct discharge to surface waters (licensed groundwater discharge is rare); or discharge to the sewer system, if one is available. Effluents standards will apply to both options: sewer regulations will require pretreatment in order to remove toxics but effluents which can be treated by normal municipal systems will be accepted, at a charge. Direct discharge standards will depend on the character and objectives of the receiving water but would normally be expected to be more stringent than sewer standards.
Because of economies of scale, sewer discharge of simple wastes such as BOD will often be cheaper than industrial on-site treatment However, there are often problems with the capacity of the municipal treatment and with implementing the correct charging systems and this option may not always be available.
Clearly, where regulations are inadequate or enforcement is lax, there is a financial incentive for industry to avoid treating the effluents.
Optimizing the Program
Once the basic information is available on water quality, municipal and industrial loads (and trends) and estimated control cost, then it is possible to begin to optimize a wastewater management program.
A key decision variable is the time scale over which the required upgrading is to be implemented. The costs of major treatment systems are so high that upgrading almost always has to be staged: at the same time, with high urban growth rates, significant investment is often required just to maintain present levels of service to the growing population. Implementation of effective industrial pollution control programs also take time and a realistic approach to projecting load reductions must be adopted.
An iterative planning process is therefore required which examines a number of different options for the scale and rate of wastewater treatment improvement, balancing the costs of the program against the time taken to achieve the water quality objectives. This process should involve an appropriate level of public discussion so that a practical program can be developed which will have the broad public and political support necessary for implementation.
Benefits and Costs
A set of agreed Water Quality Objectives (WQOs) which have been adopted by the government can be taken as reflecting the value of improving the receiving water quality, assuming that it is based on evaluation of the economic benefits of the improved uses of the water resources and also on the outcome of a public priority setting process.
The major components of a wastewater management plan, which typically compete for investment funds are:
- upgrading of sewer systems in existing urban areas to remove pollution from neighborhoods and to reduce uncontrolled discharges to local watercourses and groundwater;
- upgrading of municipal treatment systems to reduce the impacts of the effluent discharges on the receiving waters;
- introduction of a system to identify and regulate discharges from industry;
- reduction of current industrial pollution loads through recycling, improved waste management, on-site treatment or connection to sewer systems;
- adequate provision of sewerage and treatment for new urban development;
- effective control of effluent discharges from projected new industrial development;
- development of programs to quantify and tackle non-point sources of pollution, including combined sewer overflows.
The costs of these components must be considered in terms of both the overall costs and the distribution of those costs. In this way an estimate can be made of the most cost effective investments to achieve the WQOs. In effect, a marginal cost curve can be developed for the water quality improvements, although there are always many uncertainties in the estimates.
There are two practical problems that have to be resolved in order to prepare realistic options: the actual costs of pollutant removal for each of the different components; and the impacts of such removal on the water quality.
Unit Costs of Pollutant Reduction
Each of the components outlined above will have a different effective cost of pollutant reduction and the distribution of the burden of the costs will usually be different. For example, considering the case of biochemical oxygen demand (BOD), which is usually one of the main parameters, the following general conclusions can be drawn.
Upgrading sewer systems: can greatly reduce local pollution loads but will increase the loads at the treatment plant. It is reasonable and realistic to set domestic charge levels to cover at least this component of sewerage since it provides direct benefits to households and so it should be possible to cover investment costs out of increased revenue.
Upgrading municipal treatment: will address what is often the single largest point source of BOD in a system, where the costs of removal can be calculated quite accurately. (Sludge handling and disposal costs can be a significant element and must be included in the estimates.) As far as BOD removal is concerned, there are normally clear increasing marginal costs in moving from primary systems to secondary systems and onto advanced systems. The costs of treatment should in principle be borne by the system users (the polluters paying) but in political terms it is often very difficult to raise surcharges to the level necessary to cover the higher treatment levels because the users do not see the benefits directly. In many projects there will be some component of the treatment costs which will be borne directly by the government.
Introducing industrial pollution controls: in order to achieve reductions in industrial effluent discharges, it is necessary to have a regulatory and permitting system in place (no matter whether the system is based on standards or on charges). The costs of putting in place (or reinforcing) a system will be part of the investments necessary (but not sufficient) to achieve reductions in industrial discharges of BOD or other pollutants. The design of the system should specifically address the estimated effectiveness of achieving certain levels of reductions. This effectiveness depends on a number of factors but the number and size of polluters is clearly a key one: it is much quicker and more cost effective and to deal with a small number of large firms than many different small ones.
Reduction of industrial loads: estimates can be made of the volumes of BOD (for example) generated from industrial sources and costs of reduction, if an inventory of sources is available. Significant reductions in pollution loads can often be made at little or no net cost to industry (see Note on Cleaner Production) but there are often transaction costs (which are typically borne by the government) in achieving these. Clear priority should be given to ways of inducing waste minimization as a first step in reducing the overall loads.
In principle the costs of treating BOD loads from industrial sources should be no more than the costs of municipal treatment (because industry can, in the ideal case, choose to use the municipal sewers and pay the costs). Given the waste minimization opportunities which typically exist in industry, the marginal costs of pollution reduction should be no higher than the costs in the municipal system.
New urban development: new developments should be provided with sewerage and treatment systems adequate to meet the necessary discharge requirements. The costs should be borne by the users but in practice marginal developments are often both more expensive to service and occupied by poorer (often informal) households. Projections of development should therefore include realistic estimates of the extent and net cost of control of expanding urban areas.
New industrial development: it is much easier to enforce effluent standards on major new industrial developments than to retrofit and therefore the net cost of controlling new pollution loads can be expected to be less. In this context, it is important that the setting of water quality objectives takes into account the growth of urban and industrial activity so that realistic discharge requirements can be placed on new projects.
Non-point sources: for many pollutants, including BOD but particularly for nutrients, non-point sources provide a significant load. This category typically includes run-off from urban and agricultural land but can be broadened to include polluted small urban drains and streams where the precise sources of the pollution are too small and numerous to be readily identified. The costs of control of these sources are typically high but unfortunately the loads may be also high so that it is difficult to achieve the objectives by dealing with the points sources only. It is important therefore to try to address the extent and control costs of such sources.
From detailed analysis of the sources and the costs it is possible to estimate marginal reduction costs for the major types and locations of pollutant loads. These load reductions must then be translated into real water quality improvements.
Optimizing Load Reduction
Most large water catchments are not uniform and fully mixed and therefore load reductions will not all have the same impact on final water quality. In most cases, also, the WQO’s will vary across the catchment. It is therefore necessary to estimate (usually using a water quality model) the improvements that can be obtained with the implementation of different levels and locations of load reduction. (For some particular pollutants, such as heavy metals, the number and location of sources may be sufficiently limited that such modeling is not required.)
In this way, it is possible to identify, to an acceptable level of uncertainty, the most cost effective investments by the authorities in order to achieve the desired WQO’s. Once an initial estimate has been prepared, it is obviously possible to examine the implications of adopting more or less ambitious objectives.
On this basis it is then possible to have an informed process of discussion and agreement on a water quality plan and a wastewater management strategy and program.
The approach outlined here is standard when the problem is presented and tackled as a water quality management issue. Unfortunately, in sector projects, such as municipal services, industrial upgrading or pollution projects, the trade-offs between the different water pollution sources is sometimes not recognized.
For example, a major study of the impacts of the Vistuala River in Poland on pollution of the Baltic Sea (see References) identified a wide range of regulatory and institutional measures and possible investments. Priority investments were identified by a screening process, taking into account the size of the load, the cost-effectiveness of the actions and the impacts of different types of pollution. In this case, two perspectives were used to evaluate cost effectiveness: regional benefits at the level of the Baltic and benefits locally, to the population and environment directly affected. Most of the actions identified are cost effective at both levels but the priority ranking on cost-effectiveness can differ. The recommended priority investments are based on a balance of both local and regional rankings.
As an illustration of the ranking process, the cost of reduction loads on the Baltic varies from 8 ECU/kg for the most cost effective plant to 21 ECU/kg for the project ranked ninth.
Monitoring and Feedback
A major improvement program addressing a complex natural system will have uncertainties in the initial analysis and design. Sensitivity analysis will indicate which assumptions are critical and these should be reviewed and checked. However, the most critical management issue is to monitor the desired outcome (the ambient water quality) and to compare this with the projections used in design. Any major variations from the design predictions will then be identified and appropriate adjustments can be made.
Examples of Complexity
The value of detailed information and analysis can be demonstrated by a couple of examples, both concerning complexities which were identified early in the process and which were therefore taken into account in the detailed design.
Shanghai: Modeling of oxygen levels in the highly polluted Huangpo River in Shanghai demonstrated that oxygen depletion would be a problem even after high levels of treatment of wastewater discharges. The reason was that the treated wastes would have had very long detention times in the tidal section and would continued to degrade and remove oxygen. The conclusion was that costly high levels of treatment would not result in correspondingly high levels of water quality improvement.
Rio de Janeiro: Detailed modeling of Guanabara Bay uncovered the apparently perverse result that high levels of wastewater treatment could, in the short term, cause a deterioration in the overall water quality. The process involved cleaner water allowing algal blooms to occur (because of excess nutrients) with consequent severe water quality problems. The recommended approach involves a higher priority for nutrient reduction than originally proposed.