Multicriteria Evaluation of Alternative Wastewater Treatment Processes at Municipality Level

Sustainable Development and Planning II, Vol. 1 535

Multicriteria evaluation of alternative wastewater treatment processes at municipality level

K. P. Anagnostopoulos1, M. Gratziou2 & A. P. Vavatsikos1 1Department of Production and Management Engineering, Democritus University of Thrace, School of Engineering 2Department of Civil Engineering, Democritus University of Thrace, School of Engineering

Abstract

Wastewater treatment has been a highly developed issue during the two last decades in Greece, particular considering the need to comply with the requirements of the European Union directive 91/271. Although all participants in the decision making process generally agree on the necessity of interventions, a systematic opposition frequently emerges when the interventions are concretised and touch upon citizens’ reality in their local societies. At the same time, various approaches on centralized or decentralized wastewater management and the available treatment processes form a complex environment for sound decisions from the authorities at municipal and regional level. In this context, a rationalization of the decision-making process is required in order to deal with conflicting objectives. We propose a generic multicriteria approach, based on the Analytic Hierarchy Process (AHP), for the evaluation of alternative scenarios concerning wastewater treatment processes at municipality level. Evaluation scenarios are developed with respect to the number, size, treatment method, and location of plants. Multicriteria process selection is a part of a decision support system where location algorithms and GIS tools are combined in order to define the number of the alternatives and the location of the plants. We also discuss the retained evaluation criteria, land requirements, environmental criteria, construction and operating costs, etc. The application of this approach is illustrated through a case study. A sensitivity analysis is also performed in order to investigate how sensitive the ranking of the alternatives is to changes in the importance of the hierarchy criteria. The results obtained show that this approach is a viable tool and offers good communication with decision- maker. Keywords: AHP, multicriteria evaluation, wastewater treatment plants, appropriate wastewater treatment.

1 Introduction

In order to avoid the serious effects of wastewaters in the environment, current legislation requires an appropriate wastewater treatment, which permits the accomplishment of quality objectives in receiving waters after the effluents discharge [1–5]. This obligation had as a result the rapid increase of wastewater treatment plants (WWTPs) during the last two decades in many countries, including Greece. In order to comply with the EU legislation, the effort in Greece, up to now, was focused on the construction of treatment plants in the major cities of each prefecture. However, in many cases important environmental problems emerge in non-sewered rural areas with small population equivalent (p.e.) number where no treatment exists [6, 7]. Particular interest present settlements or agglomerations with less than 2000 p.e., that already have or they are about to have sewerage, and those that are not included to the EU legislation but should be taken into account due to sustainability considerations [9]. The planning of full-scale interventions at prefecture level becomes quickly very complex, if we take into account the number of treatment methods, restrictions in their implementation [2, 8], receivers dilution capability, the existent networks and plants, and the cost of constructing conduits to connect an agglomeration with the treatment plants. Moreover, decision making must take into consideration not only quantitative but qualitative criteria as well [6, 8, 9, 10]. In the present paper the Analytic Hierarchy Process (AHP), a well-known multicriteria method, is used in order to evaluate alternative wastewater treatment processes (WWTP) at municipality level with the use of economic, social and environmental criteria. As briefly explained in section 2, this work is part of a GIS based decision support system for the planning of wastewater facilities at prefecture level.

2 Location and allocation of the WWTPs

The proposed multicriteria evaluation is the third of a three steps procedure for the planning of wastewater facilities at prefecture level. In the first step, the agglomerations that demand wastewaters treatment, the receivers for the final discharge of the effluents and space availability are specified by spatial analyses using GIS tools. In this way alternative processes are evaluated according to their performance on serving the same amount of p.e. and to the ability to achieve the same treatment level as it is defined by receivers’ sensitiveness [1, 11, 12]. In the second step centralized or decentralized treatment options are considered. Since the cost of the needed conduits is getting outstanding as their length grows, decentralized treatment is preferable. In order to determine the number and the locations of the plants the notion of pole is introduced. A pole is defined as a single agglomeration or as an aggregation of agglomerations and the implementation of a plant for handling with the wastewaters by each pole is considered as a feasible scenario. For a catchment area where N agglomerations

WIT Transactions on Ecology and the Environment, Vol 84, © 2005 WIT Press www.witpress.com, ISSN 1743-3541 (on-line) Sustainable Development and Planning II, Vol. 1 537 are settled, the number of the possible poles varies between 1 (fully centralized treatment) and N (a treatment plant for each agglomeration). For the minimization the cost of conduits, a minisum algorithm is applied for finding the agglomeration from which the distances from all the others to a pole gets minimum. This agglomeration is poles absolutely median, and the plant is located to its neighbourhood. Finally, since every pole is considered to be a feasible scenario for the implementation of a plant, in any possible case multicriteria analysis is performed for selecting the most preferable treatment process for handling poles wastewaters. The final choice is taken in favour of the scenario with the lowest budget, as it is formed by the cost for the construction of the facilities and the needed conduits.

3 The multicriteria approach to WWTP selection

3.1 The analytic hierarchy process

Developed by T. L. Saaty, the Analytic Hierarchy Process (AHP) is a multicriteria method for dealing with complex decision-making problems in which many competing alternatives (projects, actions, scenarios) exist. The alternatives are ranked using several quantitative and/or qualitative criteria, depending on how they contribute in achieving an overall goal. AHP is based on a hierarchical structuring of the elements that are involved in a decision problem. The hierarchy incorporates the knowledge, the experience and the intuition of the decision-maker for the specific problem. The evaluation of the hierarchy is based on pairwise comparisons. The decision-maker compares two alternatives Ai and Aj using a criterion and assigns a numerical value to their relative weight. The result of the comparison is expressed in a fundamental scale of values ranging from 1 (Ai, Aj contribute equally to the objective) to 9 (the evidence favoring Ai over Aj is of the highest possible order of affirmation). Given that the n elements of a level are evaluated in pairs using an element of the immediately higher level, an n×n comparison matrix is obtained.

1 PPK 2 Pn P 1 … aa 1 12 1n 2 aP 12 11 … a2n

n 1n 11 aaP 2n … 1

Figure 1: Pairwise comparisons matrix Α of actions Ρi with respect to criterion K.

The decision-maker’s judgments may not be consistent with one another. A comparison matrix is consistent if and only if aij × ajk = aik for all i, j, k. ΑΗΡ measures the inconsistency of judgments by calculating the consistency index CI

WIT Transactions on Ecology and the Environment, Vol 84, © 2005 WIT Press www.witpress.com, ISSN 1743-3541 (on-line) 538 Sustainable Development and Planning II, Vol. 1 of the matrix. The consistency index CI is in turn divided by the average random consistency index RI to obtain the consistency ratio CR. The RI index is a constant value for an n×n matrix, which has resulted from a computer simulation of n×n matrixes with random values from the 1-9 scale and for which aij = 1/aji. If CR is less than 5% for a 3×3 matrix, 9% for a 4×4 matrix, and 10% for larger matrices, then the matrix is consistent [13].

Optimal WWTP selection

Cost Environ Social

Land Req Odor Pr Perform Sludge Pr Flow Fluc Constr Aesth Cons BOD5 Rem Oper+Main TSS Rem

TP Rem

TN Rem

S.B.R. O.D. R.B.C. W.S.P. S.F.+lag. S.F.+CL. Figure 2: Problems hierarchy.

3.2 The structure of the hierarchy

In our model, Analytic Hierarchy Process is used for the selection of the most preferable wastewater handling technology in a pole. This is achieved with the use of a four level hierarchy by which Compact sequential batch reactor (SBR), oxidation ditch (OD), rotating biological contactor (RBC), waste stabilization ponds (WSP), horizontal subsurface flow constructed wetlands with lagoon (SF+Lag) and horizontal subsurface flow constructed wetlands with chlorination (SF+CL) are evaluated with the use of three sets of criteria (Figure 2) [8, 9]. Analytically these criteria are: 1. Cost Criteria (Cost). This category consists of the cost for the required land for each alternative (Land Req), the construction cost (Constr) and the operation and maintenance costs (Oper&Main). The model is supported by a database which is supplied with the cost estimations for a range of population equivalent [3, 8, 14, 15]. 2. Environmental Criteria (Environ). Alternatives are evaluated according to the quality of the effluents using the percentage removal of biochemical oxygen demand (BOD Rem), solids in suspension (SS Rem), nitrogen (TN Rem)

WIT Transactions on Ecology and the Environment, Vol 84, © 2005 WIT Press www.witpress.com, ISSN 1743-3541 (on-line) Sustainable Development and Planning II, Vol. 1 539 phosphorous (TP Rem), , with respect to the amount of the produced sludge by each process (Sludge Pr), and finally their response ability to hydraulic flow fluctuations (FlowFluc) [2,6,16]. 3. Social Criteria (Social). Treatment plants are described as undesirable facilities and when they are located near to inhabited agglomerations give rise to neighbourhood opposition. In order to take into account these effects social criteria are also included in the hierarchy, odour problems provoked by each one of the alternative processes implementation (OdorProb) and aesthetic considerations (AesthCons) [1,7,16].

Table 1: Second and third level criteria priorities estimation.

1st Level Priorities 2nd Level Priorities 3rd Level Priorities Criteria Local Global Criteria Local Global Criteria Local Global Land Req 0,112 0,056 0,056 Cost 0,500 0,500 Constr 0,540 0,270 0,270 Oper&Main 0,348 0,174 0,174 BOD Rem 0,333 0,073 TSS Rem 0,333 0,073 Perform 0,661 0,220 TN Rem 0,167 0,037 Environ 0,333 0,333 TP Rem 0,167 0,037 Sludge Pr 0,208 0,069 0,069 Flow Fluct 0,131 0,044 0,044 Odor Prob 0,750 0,125 0,125 Social 0,167 0,167 Aesth Cons 0,250 0,042 0,042 4 Model application

The presented model is applied in Evros prefecture and more specifically at the municipality of New Vissa. Two catchment’s areas are identified by spatial analyses. In the first one agglomerations of Vissa (3190 p.e.), Kavilli (1650 p.e.) and Sterna (990 p.e.) are settled and in the second Kastanies (1430 p.e.) and Rizia (1870 p.e.). Multicriteria evaluation process is applied in the second case