K.D. Neder*, G.A. Carneiro**, T. R. Queiroz* and M.A.A. de Souza** * Water and Sewage Company of the Federal District (CAESB), SCS Qd. 04 – Bl. A – No 67/97 – Ed. CAESB – 4º andar, Brasilia, Brazil – 70300-904 (E-mail:
[email protected];
[email protected] ) ** Environmental and Civil Engineering Department of the University of Brasilia (UnB), Av. L3 Norte – Campus UnB – Ed. SG-12 – MTARH, Brasilia, Brazil – 70910-900 (E-mail:
[email protected];
[email protected] ) Abstract A multicriterion methodology is used in the evaluation and selection of the most appropriate alternative(s) for removing algae from stabilisation ponds effluents in a case study in Brasilia. For this purpose, five different natural treatment processes are tested at pilot scale: rock filter, sand filter, floating aquatic plants, constructed wetlands, and overland flow. These pilot units were constructed in Brasilia and set in parallel, each one receiving a portion of the effluent that comes from an existing wastewater treatment plant composed of preliminary treatment, UASB reactors, and high-rate stabilisation ponds. Several evaluation criteria are used in order to relate the capabilities of the post-treatment processes to the multiple objectives in this case. Two multicriterion decision-aid methods – compromise programming and ELECTREIII – are used to select the most satisfying processes. The top ranking alternatives are indicated for subsequent studies, considering the possible implementation of these technologies to existing plants. Keywords Effluent polishing; multiobjective analysis; natural wastewater treatment; process selection; stabilisation ponds
Water Science and Technology Vol 46 No 4–5 pp 347–354 © IWA Publishing 2002
Selection of natural treatment processes for algae removal from stabilisation ponds effluents in Brasilia, using multicriterion methods
Introduction
Stabilisation ponds (or lagoons) are advantageous wastewater treatment processes, especially for developing countries, since they usually demand low capital cost and present great simplicity for operation and maintenance. In addition, the climatic conditions of tropical countries, like Brazil, are favourable to the installation of this type of treatment process and most natural wastewater treatment processes. Nevertheless, in spite of the well known advantages of the implementation of the stabilisation pond processes, the effluent of this type of treatment system usually has high concentrations of suspended solids (SS), mainly due to the significant amounts of algae in it (Luduvice et al., 2001). In order to solve this inconvenience – which can be harmful to the receiving waters and can hinder the water reuse for a wide range of different applications – it is necessary to look for post-treatment processes that can provide significant removal of SS and, at the same time, assure that the treatment system as a whole will maintain the primordial advantages of the pond treatment processes (i.e. low cost and operational simplicity). In this context, the present research intends to develop a multicriterion study that can evaluate and select the most suitable alternative(s) among different natural wastewater treatment processes capable of being used to remove suspended solids from stabilisation ponds effluents (Neder et al., 2000). For this purpose, five natural post-treatment processes were chosen for evaluation, as shown in Table 1. For each proposed process a pilot unit was constructed in the boundaries of an existing wastewater treatment plant in Brasilia (ETE-Paranoa), which is composed of preliminary
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Table 1 Natural post-treatment processes evaluated Alternatives
K.D. Neder et al.
A– B– C– D– E–
Rock filter Intermittent sand filter Floating aquatic plants (Eichhornia crassipes) Constructed wetlands – subsurface flow system (Typha sp.) Overland flow
treatment, UASB reactors and high-rate stabilisation ponds, and treats wastewater coming from a community of approximately forty five thousand inhabitants (Neder et al., 2001; Queiroz, 2001). The five pilot units tested were set in parallel, each one receiving a portion of the effluent that comes from a high-rate stabilisation pond of the treatment plant previously mentioned. These units had been monitored for over a year, with several water quality parameters regularly analysed, and operational conditions observed (Queiroz, 2001). The following evaluation methodology takes into account the different dimensions of such a multiobjective decision-making problem, which are represented by several evaluation criteria. These criteria were established in order to relate the capabilities of the post-treatment processes to the multiple objectives in this case study. Each evaluation criterion was given an importance-weighting factor. The attributes of the evaluation criteria were estimated based on the experimental data obtained at the pilot units. These attributes are presented together in an evaluation matrix called the “payoff matrix”. However, if the payoff matrix becomes too large, the greater will be the difficulties to intuitively combine evaluation data to select the most appropriate alternative(s). In this way, multicriterion techniques can be very useful as tools that can help the decisionmaking process. Two different multicriterion decision-making techniques were used to analyse and select the most satisfying alternative(s) for this case. These techniques – the compromise programming and the ELECTRE-III methods – can both analyse the set of proposed feasible alternatives by means of simultaneously using the set of multiple discrete evaluation criteria (Goicoechea et al., 1982). Objectives
The major objective of the present research is to evaluate and select the most appropriate post-treatment process(es) for algae removal, among five different natural treatment alternatives monitored at pilot scale, which receive the effluent of a stabilisation pond unit that is part of a real scale wastewater treatment plant in Brasilia, Brazil. The multicriterion decision-making methodology proposed can also be used as a reference in the selection of alternatives for polishing stabilisation ponds effluents in different situations. Methodology
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Prior to choosing the post-treatment alternatives to be evaluated, the multiple objectives of this decision-making problem were specified. First, it was defined that, besides relevant suspended solids (SS) removal, the post-treatment process desired should present operational simplicity, low costs, easy implementation, and good overall treatment efficiency. Therefore, natural treatment systems were elected for further evaluation. This type of treatment process is intended to reach the specified objectives, taking advantage of the physical, chemical, and biological treatment processes that occur “naturally” in a single “ecosystem reactor” (Metcalf and Eddy, 1991).
K.D. Neder et al.
Among the great amount of natural treatment process possibilities for SS removal, the most promising alternatives, according to the literature, were chosen for deeper research (Wolverton and McDonald, 1979; McDonald and Wolverton, 1980; Reed et al., 1988; EPA, 1993; Middlebrooks, 1995; Saidam et al., 1995). These alternatives are presented in Table 1. Pilot units were constructed and operated in agreement with the design dimensions and operational conditions shown in Table 2. These values were established based on previous experience, in order to reach significant SS percent removal (Queiroz, 2001). The overland-flow pilot unit was covered with a common local type grass. For the other units, an earthen basin was excavated, with the dimensions aforementioned, and coated with a 5.0 cm concrete layer. Preliminary observations, and operational adaptations were performed during the first year of operation of the post-treatment processes. This period also provided support for the definition of the hydraulic-loading rates to be used in this research. Subsequently, the posttreatment alternatives were monitored with respect to several water quality and operational parameters, on a regular basis, for a two-month period. Several criteria were established in order to evaluate the performance of each alternative in terms of the intended objectives previously specified. Weighting factors were defined for each evaluation criterion according to its degree of importance in the whole evaluation procedure (Table 3). The task of choosing the most preferred alternative(s), considering the set of evaluation criteria altogether, was found too complex to be performed without any type of formal assistance. On the other hand, ignoring relevant evaluation criteria, in order to simplify the decision-making problem, was not considered a reasonable solution. Therefore, two multicriterion techniques were chosen to assist the evaluation process: compromise programming and ELECTRE-III methods. The selection of these techniques was based on previous applications to similar case studies, on the adaptation feasibility to the current problem, and on its ease of use. The first technique, called compromise programming, is a distance-based method designed to measure the proximity of the alternatives to an ideal solution (Zeleny, 1973, 1974). In this technique, each alternative is evaluated separately using appropriate distance patterns to the ideal situation (represented by the set of the best performances related to each criterion). Duckstein and Opricovic (1980) presented a version of this method that can be easily applied to numerical discrete problems. Tecle et al. (1988) performed an application of this multicriterion method to a similar case study. The ELECTRE-III is one of ELECTRE series methods, named after the French denomination for “elimination and (et) choice translating reality” (Benayoun et al., 1966; Roy, 1971, 1991). This technique can perform pair-wise comparisons, among proposed alternatives, to determine their partial ranking. It is a type of successive elimination method that highlights the most preferred alternatives in each criterion. These outranking relations are Table 2 Installation and operation characteristics of the post-treatment units Alternatives (simplified
Hydraulic-l
Operating
denomination)
oading rate
cycle
(m3/m2.day)
(hours)
Length
Width
Depth
Media layer (m)
0.024 0.10 0.024 0.024 0.016
24 12 × 12 24 24 12 × 12
15.00 6.00 15.00 15.00 30.00
3.80 4.50 3.50 3.80 3.00
0.80 1.05 0.80 0.80 –
0.60 0.45 0.30* 0.60 –
A– B– C– D– E–
Rock filter Sand filter Aquatic plants Wetlands Overland flow
Dimensions (m)
Water depth*
Slope (%)
(m) or
– – – – 4
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Table 3 Objetives, criteria, and criterion scales Objectives
K.D. Neder et al.
Relevant suspended solids (SS) removal Operational simplicity Low costs Easy implementation Good overall treatment efficiency
Evaluation criteria
Criterion
Preference
scales
order
Weighting
SS removal efficiency
%
Increasing
3
Operation and maintenance cost Capital cost Land area required Implementation feasibility to existing pond treatment systems in Brasilia BOD5 removal efficiency
R$/person R$/person m2/person Ordinal (1–5)
Decreasing Decreasing Decreasing Increasing
2 2 1 1
%
Increasing
1
TKN removal efficiency Total P removal efficiency NO2 + NO3 removal efficiency Effluent quality variation Odour and mosquitoes detection
% % % Rational [1, 5] Ordinal (1–5)
Increasing Increasing Increasing Increasing Increasing
1 1 1 1 1
factor
then used to construct two pre-orders: an increasing – going from the least to the most preferred alternatives in all criteria; and a decreasing – defined in the inverse direction. A final ranking is established as a result of the intersection of these two pre-orders. Duckstein et al. (1994) used this method in a correlated area. Further description on the application procedure of these multicriterion techniques can be found in the above-mentioned references. The evaluation procedure was then divided into two stages. As a first step, it was intended to test and adapt the multicriterion techniques to this specific case. Factors related to the construction of the evaluation matrix were also observed. In the next phase, the final payoff matrix was elaborated and the decision-aid techniques were used to select the most satisfying alternatives to this case. These two stages and their respective results are presented in the following paragraphs. First stage procedure: preliminary evaluation
For the preliminary evaluation procedure, six criteria among all present in Table 3 were chosen as follows: (i) suspended solids (SS) percent removal; (ii) biochemical oxygen demand (BOD5) percent removal; (iii) total Kjeldahl nitrogen (TKN) percent removal; (iv) total phosphorus (Total P) percent removal; (v) nitrate plus nitrite (NO3+NO2) percent removal; and (vi) operation and maintenance cost. The evaluation of all alternatives with respect to these criteria formed a payoff matrix [i × j]. In this matrix, the rating of the ith criterion on the jth alternative (i = 1,2,. . .,m and j = 1,2,. . .,n) is represented by Rij. For the percent removal criteria, the ratings of the alternatives were calculated by using the following equation: Rij (%) = 100 × (Ii – Eij)/Ii
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(1)
where: Rij = the evaluation of the jth alternative with respect to the percent removal of the pollutant considered in the ith criterion Ii = influent concentration, for all alternatives, of the pollutant considered in the ith criterion Eij = effluent concentration, for the jth alternative, of the pollutant considered in the ith criterion
The operation and maintenance costs, on the other hand, were calculated for each alternative by estimating the total manpower work hours dedicated to the routine operation and maintenance activities, plus the average extra intervention activities in a month. This data was converted to local monetary values (R$) and divided by the equivalent number of persons that would contribute to the hydraulic-loading rates used in each unit. For the overland-flow unit, the mechanical mowing activities were also considered.
The evaluation procedure performed in the first stage resulted in the construction of the payoff matrix presented in Table 4. The expert group involved in this research performed an informal evaluation of the proposed alternatives based on the data presented in this matrix. This task was found to be rather complicated, even though the payoff matrix cannot be considered very large. Such activity, thus, indicated that global judgments are not as accurate as analytical evaluations. This evaluation selected alternatives A and C (respectively rock filter and aquatic plants) as the most preferred. The major factor that contributed to this decision was the high SS percent removal reached by both alternatives. Besides, these two processes presented intermediate to high performances on the percent removal of organic matter and nutrients. Alternative D (wetlands) also reached a high SS percent removal. However, this process presented low performances in removing phosphorus and nitrogen, in addition to the high cost demanded for routine operation and maintenance activities. Nitrate and nitrite percent removal was found to be an inadequate form of analysing the processes treatment efficiency. This was due to the fact that the transformation and removal of nitrogen in natural systems involve a complex set of processes and reactions that cannot be summarized by simply using this evaluation criterion. Alternatives B and E (respectively sand filter and overland flow) were considered the least preferred in this evaluation. These two processes presented intermediate to low performances in all criteria, except for O & M costs, where alternative B demanded the lowest cost. Compromise programming and ELECTRE-III methods were then applied to this payoff matrix. The use of these multicriterion evaluation techniques provided the results shown in Table 5. A unique ranking of alternatives was obtained by both multicriterion methods. This classification agreed with the previous evaluation, where alternatives A and C were considered the most satisfying, and alternatives B and E the least preferred ones. Thus, the applicability of these methods to the current problem was confirmed.
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First stage results and discussion
Second stage procedure: final ranking
A new payoff matrix was constructed in the final evaluation stage. This matrix includes all evaluation criteria presented in Table 3, except for the NO3 + NO2 percent removal, and Table 4 1st stage payoff matrix Criteria
SS removal efficiency (%) BOD5 removal efficiency (%) TKN removal efficiency (%) Total P removal efficiency (%) NO3 + NO2 removal efficiency (%) Operation and maintenance cost (R$/person)
Weight
3 1 1 1 1 2
Alternatives A
B
C
D
E
95.00 84.25 13.51 21.34 13.6 0.07
44.97 17.83 27.81 21.59 –1878 0.01
80.70 53.10 41.23 49.75 –49.5 0.34
91.07 85.95 1.71 13.18 85 0.91
31.81 1.63 20.42 20.48 –894 0.38
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Table 5 Preliminary outranking of alternatives according to the multicriterion methods Rank
Compromise
Electre-III
programming
K.D. Neder et al.
1st 2nd 3rd 4th 5th
A C D B E
A C D B E
odour and mosquito detection criteria. This is because the first criterion was not found to be a reasonable evaluation tool, as explained earlier. For the other criterion, all processes had the same performance: no odour or mosquitoes were detected. The capital cost was calculated by considering all necessary materials and activities for installing the pilot processes. The land area demanded was measured directly at the pilot units. In both cases, the attributes were presented in terms of equivalent per capita values (R$/person and m2/person). Implementation feasibility was estimated for each alternative by considering its installation characteristics and the conditions of the existing pond systems in Brasilia. In this way, an arbitrary ordinal scale was established, from 1 to 5, directly proportional to the performance of each alternative. This grading system takes into account the estimated amount of activities required to adapt the existing plants for the implementation of these post-treatment facilities. The possibility of further reversibility is also considered. The variation of the effluent quality was estimated by calculating the standard deviation of SS and BOD5 effluent concentrations, for each alternative, under the same operational conditions, and during the same time period. The obtained data was then compiled and converted to a rational scale, within the interval [1, 5] where the less reliable the process the smaller its gradation. The other criteria that were already tested in the preliminary payoff matrix were maintained unaffected. Second stage results and discussion
The results of the final evaluation are presented in Tables 6 and 7.
Table 6 2nd stage payoff matrix Criteria
SS removal efficiency (%) BOD5 removal efficiency (%) TKN removal efficiency (%) Total P removal efficiency (%) Operation and maintenance cost (R$/person) Capital cost (R$/person) Land area required (m2/person) Effluent quality variation (*) Implementation feasibility to existing pond treatment systems in Brasilia (*)
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Weight
3 1 1 1 2 2 1 1 1
Alternatives A
B
C
D
E
95.00 84.25 13.51 21.34 0.07 185.19 5 5 4
44.97 17.83 27.81 21.59 0.01 73.27 1.2 1 3
80.70 53.10 41.23 49.75 0.34 125.38 5 1.5 5
91.07 85.95 1.71 13.18 0.91 198.48 5 4.75 3
31.81 1.63 20.42 20.48 0.38 80.5 7.5 1.25 2
(*) The attributes presented in the matrix for these two criteria were rated according to pre-defined scales of measurement, as it was explained earlier. In both cases, the values are directly proportional to the preference intensity
Table 7 Final outranking of alternatives according to the multicriterion methods Rank
Compromise
Electre-III
programming
C A B D E
A C and B D E –
Conclusions
K.D. Neder et al.
1st 2nd 3rd 4th 5th
Alternatives A and C (respectively rock filter and aquatic plants) were considered the most preferred ones since they appeared in the 1st and 2nd places of the ranking scale, both in the preliminary and final evaluations. A comparison between the evaluations of the preliminary payoff matrix and the final one shows that a more sophisticated multicriterion evaluation process can show results that do not necessarily appear at first glance. In fact, alternative B (sand filter), which obtained a weak overall performance in the first stage evaluation, appeared at a later stage in a better ranking. This was due to the fact that as new criteria were used new relative advantages were found. The multicriterion methodology used helped in the decision-making process in the sense that more appropriate solutions were found for the case study, taking into consideration the multiple objectives delineated in the current problem. For instance, rock filter and aquatic plants processes have been used subsequently in more and deeper studies. New projects are currently been developed for the diffusion of these technologies, including the possible implementation of one of them to upgrade the pond treatment systems existing in Brasilia. It is important to highlight that the use of these multicriterion decision-making techniques allows for practical work within a multidisciplinary context. This may be helpful in finding more appropriate solutions for specific situations in sanitary systems management and correlated areas. References Benayoun, R., Roy, B. and Sussman, B. (1966). ELECTRE: Une Méthode pour Guider le Choix en Presence de Points de Vue Multiples, Note de Travail 49, SEMA-Direction Scientifique, Paris, France. Duckstein, L. and Opricovic, S. (1980). Multiobjective optimization in river basin development. Water Resources Research, 16(1), 14–20. Duckstein, L., Treichel, W. and Magnouni, S.E. (1994). Ranking ground-water management by multi criterion analysis. Journal of Water Resources Planning and Management, 120(4), 546–565. EPA (1993). Subsurface Flow Constructed Wetlands For Wastewater Treatment: A Technology Assessment. United States Environmental Protection Agency, Washington, DC, USA, 66 p. Goicoechea, A., Hansen, D.R. and Duckstein, L. (1982). Multiobjective Decision Analysis with Engineering and Business Applications. John Wiley & Sons, New York, USA, 519 p. Luduvice, M.L., Neder, K.D., Queiroz, T.R. and Souza, M.A.A. (2001). Sólidos suspensos como indicador da densidade de algas em lagoas de estabilização. 21° CABES, João Pessoa, Brasil, 2001. McDonald, R.C. and Wolverton, B.C. (1980). Comparative study of wastewater lagoon with and without water hyacinth. Economic Botany, 34(2), 101–110. Metcalf & Eddy, Inc. (1991). Wastewater Engineering: Treatment, Disposal, Reuse. 3rd edn, McGraw-Hill International Editions, New York, USA, 1334 p. Middlebrooks, E.J. (1995). Upgrading pond effluents: an overview. Wat. Sci. Tech., 31(12), 353–368. Neder, K.D, Queiroz, T.R. and Souza, M.A.A. (2000). Remoção de sólidos suspensos de efluentes de lagoas de estabilização por meio de processos naturais. XXVII Congresso AIDIS, Porto Alegre, Brasil, 2000. Neder, K.D., Queiroz, T.R. and Souza, M.A.A. (2001). Utilização de processos naturais para polimento de efluentes de lagoas de estabilização. 21° CABES, João Pessoa, Brasil, 2001.
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