PERFORMANCE ANALYSIS OF THE WASTEWATER TREATMENT PLANTS USING A METABOLISM MODEL. Rodrigo Ribeiro Rego. Extended Abstract

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1 PERFORMANCE ANALYSIS OF THE WASTEWATER TREATMENT PLANTS USING A METABOLISM MODEL Rodrigo Ribeiro Rego Extended Abstract September 2012

2 Abstract The urban growth observed as a result of the development of the modern societies is accompanied by concerns in the water sector, as a result of the increasing requirements for water supply and wastewater treatment. This situation justifies the evaluation of the system performance that covers protection of water resources, management of infrastructure and also all the different flows of materials and energy. The present work includes the development of a metabolism model of WWTP with different treatment systems, comprising the identification of material flows and energy and a quantification of costs. The flows considered for each WWTP included the pollutants present in the effluent, the production of sludge and waste, the reagents used in the treatment process, the greenhouse gases emissions and the energy associated with the operation of WWTP. The value of each flow was determined per unit volume of affluent water in order to allow for comparisons between treatment plants of different sizes. The costs associated with the treatment of a unit volume of wastewater were calculated by adding the specific costs of operation, according to the previously determined flows, and the specific cost of investments in fixed capital in the WWTPs (p unit ). This approach allows the internalization of externalities as the greenhouse gases impact, which are generally not accounted for in economic analyses of water services. The methodology was applied to a case study in the service area of Águas do Algarve, including the analysis of WWTP with treatment systems by activated sludge and trickling filters. Keywords: performance, metabolism model, activated sludge, trickling filters. 1. Introduction The urban growth observed as a result of the development of the modern societies is accompanied by concerns in the water sector, as a result of the increasing requirements for water supply and wastewater treatment, which build up an increasing pressure on water resources. In addition, factors such as infrastructure aging and climate change are also sources of pressure on the services of the urban water cycle, justifying the evaluation of the system performance that covers protection of water resources, management of infrastructure and also all the different flows of materials and energy. The wastewater treatment is a crucial process for the environmental balance nevertheless it features a high consumption of resources and energy as well as a high production of waste and emissions. The application of metabolism models to the water services is a tool that allows 1

3 studying separately the energy flows and mass of the system, thus giving a perception of associated products. The present work includes a description of a metabolism model applied to Wastewater Treatment Plants, identifying the input (pollutant loads, clean water, reagents and energy) and output flows (pollutant loads, sludge, waste and gas emissions). The model was applied to several WWTP that operate in the Algarve region, with the goal of determining the byproducts of these treatment plants and possible areas of intervention, in order to optimize the system. The methodology developed further encompasses the accounting of costs associated with the treatment of a unit volume of wastewater being calculated by adding the specific costs of operation, according to the previously determined flows, and the specific cost of investments in fixed capital in the WWTPs (p unit ). This approach allows the internalization of externalities as the greenhouse gases impact, which are generally not accounted for in economic analyses of water services. 2. Methodology 2.1 Performance analysis of the WWTPs The methodology developed to study the performance of the WWTP is divided into the following steps: Identification and characterization of flows associated to the operation of WWTP, namely: Pollutants Biochemical Oxygen Demand (BOD), Chemical Oxygen Demand (COD), Total Suspended Solids (TSS), Nitrogen, Phosphorus; Water consumption from the public network; Energy consumption; Reagents consumption; Sludge production; Waste production; Greenhouse gases emissions associated to energy consumption and wastewater treatment. Determination of the specific consumption indicator and specific production for each identified parameter, by dividing the annual flow of a given parameter by the affluent flow rate, thus facilitating the comparison between treatment plants of different sizes. The determination of the pollutant loads present in the wastewater entering and leaving the WWTP was carried out monthly, considering the monthly average flow rate and the monthly average concentration of each pollutant, being the annual value given by the sum of all monthly values. The consumption and production of the remaining identified parameters in the metabolism model was determined by summing the values obtained monthly. In the development of this case study it was adopted a more simplified notation for the greenhouse gases (GHG). Thus, the GHG emissions due to the electric energy consumption 2

4 are referred as GHG energy and the GHG emissions due to the wastewater treatment are referred as GHG operation Determination of total specific costs Specific cost of operation The methodology adopted for the determination of the specific costs of operation was developed in the following way: Determination of the annual costs of parameters associated with the operation phase of each WWTP, namely: Tax on the discharge of treated effluent; Drinkable water consumption; Electric energy consumption; Reagents consumption; Sludge production; Waste production; GHG emissions from the operation. Calculation of annual specific costs per m 3 of treated water, of each of the identified parameters ( /m 3 treated water); Determination of the total specific cost of operation of each WWTP, by summing the specific costs of operation of the different parameters considered at this stage. The determination of these costs will not take into account aspects such as staff costs, transport, fuel and facilities which are also associated with normal operation of the stations Specific cost of investments in fixed capital in the WWTPs (p unit ) The determination of the specific cost of investments in fixed capital in WWTPs will be developed according to the following steps: Determination of a flow rate evolution for each WWTP, until the horizon year of the project; Correction of the investment made in each WWTP, taking into account the year where the investment was made and the coefficients of devaluation of currency; Update the corrected investment to a reference year, to a fixed update rate; Determination of the annual costs of investment resulting from the imputation of m 3 of treated water, to a reference year, until the horizon year of project; Determination of specific costs of investment in fixed capital (p unit ). 3. Results and Discussion The present study focused on operational data for the year of 2010, being the methodology described in the previous chapter applied to the WWTP of Vilamoura, Loulé, Olhão and Quinta do, located in the municipalities of Loulé and Olhão, Algarve. All WWTP have a tertiary treatment level of the effluent (UV disinfection). The WWTP of Olhão and Quinta do also have a nutrient removal stage. 3

5 The main characteristics of each WWTP studied are presented in Table 1. Table 1 Treatment system, population in the horizon year of project and starting year of the WWTPs under study. Treatment system WWTP Liquid phase Solid phase Gas phase Population in the horizon year of project (eq.) Starting year (transfer of the WWTPs to AdA) Vilamoura Activated sludge, trickling filters, UV disinfection and maturation pond Thickener, belt press filter Scrubber and biofilter Loulé Activated sludge by oxidation ditch system and UV disinfection Thickener, belt press filter Olhão Activated sludge, removal of nitrogen and UV disinfection Thickener, belt press filter Deodorization tower Quinta do Activated sludge by oxidation ditch system and aeration tank (medium load), removal of nitrogen and phosphorus and UV disinfection Thickener, digester and belt press filter Deodorization tower The values of affluent flow rate of the last years on the WWTPs under study, the flow rate at the horizon year of the project and the respective year are presented in Table 2. Table 2 Affluent flow rates between 2007 and 2011, flow rate and population on the horizon year of project of the WWTPs under study. Flow rate (m 3 /year) WWTP/Year Flow rate at the horizon year of project Horizon year of project Vilamoura Loulé Olhão Quinta do Performance analysis of the WWTPs The affluent flow rate values to each WWTP coincide with the flow rate of treated water, because the consumption of drinkable water in the facilities takes very small values (0.5% of affluent flow rate). The annual affluent flow rate and the equivalent population served by the WWTPs, in the year of 2010, are presented in Table 3. 4

6 Specific energy consumption (kwh/m 3 ) Table 3 Affluent flow rate and equivalent population served by WWTPs under study, in the year of WWTP Annual Flow Rate (m 3 ) Equivalent population served (inhab.) Vilamoura Loulé Olhão Quinta do Figure 1 shows the specific energy consumption of the WWTPs under study. The WWTP of Quinta do is the station that has the highest specific consumption, value that is associated to the stage of nitrification/denitrification by biofilters, which requires continuous aeration and washings. 1,20 1,00 0,98 0,80 0,60 0,40 0,72 0,78 0,68 0,20 0, Equivalent population served (inhab.) Figure 1 Specific energy consumption of the WWTPs under study. It should be noted that in the WWTP of Vilamoura and Loulé the disinfection step is only put into operation during 5 or 6 months in a year, respectively, factor which is responsible for the decrease of the specific consumption of electricity in these WWTP. The specific sludge production on the WWTPs under study is displayed in Figure 2. The WWTP of Loulé is the station that has a higher value, which might be related to the high pollutant loads of COD and BOD it receives monthly, due to discharges of oil from septic tanks, solvents and inks during the first ten months of the year and the discharges of effluent from the industry of medronho processing during the month of April. 5

7 Specific GHG production (kg CO 2 e/m 3 ) Specific sludge production (kg/m 3 ) 2,50 2,00 2,04 1,50 1,00 1,20 1,19 1,05 0,50 0, Equivalent population served (inhab.) Figure 2 Specific sludge production on the WWTPs under study. The specific greenhouse gases production in our system is accounted as the sum of two parts: GHG energy and GHG operation (Figure 3). In this field there was no scaling effect, since the specific emission of each WWTP has a value similar to the other plants. 1,20 1,00 0,80 0,60 0,40 1,00 0,75 1,05 0,96 0,20 0, Equivalent population served (inhab.) Figure 3 Specific greenhouse gases production on the WWTPs under study. Figure 4 shows the final diagrams of the metabolism model applied to the WWTPs under study. In these figures, the flow of electrical energy is expressed in the form of energy consumption (kwh/m 3 affluent water) and CO 2 e emissions (kg/m 3 affluent water), being that only one of these variables must be considered in the flow analysis, depending on the selected unit. 6

8 Emission due to the Wastewater Treatment Specific production (kg CO 2 e/m 3 ) 0,79 Affluent Wastewater Flow (m 3 ) TSS (kg/m 3 ) 0,204 COD (kg/m 3 ) 0,554 BOD 5 (kg/m 3 ) 0,325 N T (kg/m 3 ) 0,062 Flow (m 3 ) P T (kg/m 3 ) 0,007 TSS (kg/m 3 ) 0,017 COD (kg/m 3 ) 0,047 Water from public network BOD 5 (kg/m 3 ) 0,008 Specific consumption (L/m 3 ) 0,60 N T (kg/m 3 ) 0,013 Energy Specific consumption (kwh/m 3 ) 0,68 Specific production (kg CO 2 e/m 3 ) 0,16 Treated Wastewater P T (kg/m 3 ) 0,003 Reagents Specific consumption of polyelectrolyte (g/m 3 ) 1,86 Specific production (kg/m 3 ) 1,05 Total production (ton) 4.549,71 Agriculture application (ton) 1.454,00 Landfill (ton) 1.428,78 Composting (ton) 1.666,93 Specific production (kg/m 3 ) 0,03 Figure 4 Metabolism model associated to the. Sludge Waste 3.2. Determination of total specific costs Specific cost of operation After determining the flows associated to the functioning of the WWTPs under study, it was possible to account for the total operation costs, taking into account the described parameters in the methodology of work and associated costs. After obtaining these costs the specific costs of operation of each WWTP were determined, in order to facilitate the comparison between different WWTP (Figure 5). 7

9 p unit ( /m 3 ) Specific cost of operation ( /m 3 ) 0,250 0,200 0,185 0,191 0,150 0,100 0,168 0,137 0,050 0, Equivalent population served (inhab.) Figure 5 Specific cost of operation of the WWTPs under study. The WWTP of Loulé is the one that has a higher specific cost of operation, which might be related to the high pollutant loads that it receives monthly, leading to a higher consumption of reagents and sludge production. In turn, the WWTP of Vilamoura, although being the station that serves a larger number of equivalent inhabitants, it s the one with the lower specific cost of operation Specific cost of investments in fixed capital in WWTPs (p unit ) In the development of the case study it was also determined the unitary cost per m 3 of treated water, taking into account the investments in fixed capital in WWTPs. The presentation of the results on this field will be made according to served population in the horizon year of project of each WWTP, since the study of the annual costs of investment extends to the horizon year of project of each station. The p unit values obtained for the WWTPs under study, considering an update rate of 4%, are shown in Figure 6. The is the station that has a greater value of p unit and the the lowest value. 0,300 0,280 0,250 0,200 0,150 0,100 0,171 0,113 0,050 0,051 0, Population in the horizon year of project (eq.) Figure 6 P unit values of the WWTPs under study, for the update rate of 4%. Table 4 presents the values of p unit obtained to the update rates of 6% and 8%. In this case the continues to be the station that has a greater value, followed by the. 8

10 Total specific cost ( /m 3 ) Table 4 P unit values of the WWTPs under study, for the update rate of 6% and 8%. p unit ( /m 3 ) WWTP Vilamoura WWTP Quinta do t a=6% 0,135 0,055 0,355 0,179 t a=8% 0,159 0,060 0,436 0, Total specific cost The determination of the total specific costs is obtained by summing the specific costs of operation and the specific costs of investments in fixed capital in the WWTPs. The presentation of these results will be made in accordance with the update rates considered in the determination of p unit and for a constant specific cost of operation, for the year of The total specific costs, for an update rate of 4% are shown in Figure 7. It was decided to represent the total specific costs according to population served in the horizon year of the project, assuming that the specific cost of operation for the different years of operation is constant, since it is presented per unit volume of treated wastewater ( /m 3 ). 0,500 0,450 0,400 0,350 0,300 0,250 0,200 0,150 0,100 0,050 0,000 0,448 0,356 0,241 0, Population in the horizon year of project (eq.) Figure 7 Total specific cost of the WWTPs under study, for the update rate of 4%. As can be seen by the analysis of Figure 7, the continues to be the station that presents higher results compared to the remaining stations, value that is due to the p unit value. On the other hand, the is the station that presents the lower value of this cost, however, already very close to the value presented by the. The total specific costs for the update rate of 6% and 8% are shown in Table 5. These costs have only an increase in the component of p unit, keeping the specific cost of operation constant. Table 5 Total specific cost of the WWTPs under study, for the update rate of 6% and 8%. Total specific cost ( /m 3 ) t a=6% 0,272 0,246 0,523 0,363 t a=8% 0,296 0,250 0,604 0,371 A sensitivity analysis of the was also developed, focusing on the extension of the operation phase of this WWTP from 2010 to 2016 and determining the value of p unit corresponding to this change. The result was significant, showing a reduction of, 9

11 approximately, 0,100 /m 3 on the p unit values obtained for a phase of operation through 2010, for the update rates analyzed. 4. Conclusions The application of metabolism models to water services can be an important tool for the separate study of energy flows and mass of the system, thus allowing a phased perception of the associated byproducts and possible areas of intervention in order to optimize the system according to the desired objectives. This model was applied to four WWTP of Águas do Algarve, namely the WWTP of Vilamoura, Loulé, Olhão e Quinta do, for the year of In order to compare the WWTP under study, that display different dimensions and treatment lines, the value of each flow was determined per unit volume of affluent water. The methodology developed took also into account the total specific costs associated to the treatment of a unit volume of wastewater. These costs were accounted for by summing the specific cost of operation, according to the previous determined flows, and the specific cost of investments in fixed capital on WWTPs (p unit ). The specific energy consumption by the WWTP presents itself similar for all of the plants, with the exception of the WWTP of Quinta do which displays the highest value, of, approximately, 1 kwh/m 3 affluent. This value is related with the stage of nitrification/denitrification by biofilters that demand continuous aeration and washings. The specific sludge production is similar in all the WWTPs, with the exception of the WWTP of Loulé, that displays a value, of approximately, twice as much as the remaining plants. This value is associated with the high polluting loads that the WWTP of Loulé receives monthly, due to discharges of oil from septic tanks, solvents and inks during the first ten months of the year and the effluent discharges from the industry of medronho processing during the month of April. The production of GHG by the WWTPs under study considers the sources of GEE energy and GEE operation. This production displays similar values for the different studied WWTP, showing that there was no scaling effect in this field, whereby, when the produced emissions are calculated in terms of specific emissions, the trend remains. The specific cost of operation of the WWTPs under study displayed similar values for the WWTP of Olhão and Loulé, being the last the one that has a higher value (0,191 /m 3 ). In its turn, the WWTP of Vilamoura is the one that shows a lower value of this cost (0,137 /m 3 ). It is important to note that the operation costs did not include some costs which can have a significant role in the operation phase, such as staff costs, transport, fuels and facilities. The obtained p unit values, for the update rate of 4%, are very different between the highest value, 0,280 /m 3 for the WWTP of Olhão and the lowest value, 0,051 /m 3 for the WWTP of Loulé. If the update rate is changed to 6% and 8%, the p unit values increase being the WWTP of Olhão e Quinta do the stations with highest values. 10

12 The total specific cost obtained for the WWTPs under study, for the update rate of 4%, displays different values from station to station, being the WWTP of Olhão the one that obtained a higher value, 0,448 /m 3, and the WWTP of Loulé the one that showed a lower value, 0,241 /m 3. With the application of higher rates, the values of the total specific cost will also be higher, under the influence of the p unit growth. The WWTP of Olhão displays the highest values, for the update rates of 6% and 8%, reaching values of 0,523 /m 3 and 0,604 /m 3, respectively. References AdA. (2010). Relatórios de Exploração do Subsistema de Águas Residuais de Vilamoura. Município de Loulé. AdA. (2010). Relatórios de Exploração do Subsistema de Águas Residuais de Loulé. Município de Loulé. AdA. (2010). Relatórios de Exploração do Subsistema de Águas Residuais de Olhão. Município de Loulé. AdA. (2010). Relatórios de Exploração do Subsistema de Águas Residuais de Quinta do. Município de Loulé. APA. (2012). Portuguese National Inventory Report on Greenhouse Gases, CdC climat research. (2011, January). Tendances Carbone. The Monthly Bulletin on the European Carbon Market. N.º 54. EDP. (2012). Alterações Climáticas. (Acedido em ). IPCC. (2006). Guidelines for National Greenhouse Gas Inventories. IPCC. (2001). Good Practice Guidance and Uncertainty Management in Greenhouse Gas Inventories. PNAC. (2006). Programa Nacional para as Alterações Climáticas. Anexo Técnico. Resíduos. Portaria n.º 282/2011. Diário da República 1ª Serie - N.º de Outubro de Ministério das Finanças. 11