Significance of Substrate Selection in the Efficiency of Wastewater Treatment in Constructed Wetlands (CWs) ()
1. Introduction
Nature-derived solutions are both natural and constructed systems that utilize and reinforce physical, chemical and microbiological treatment processes (Sean O’Hogain, 2018) [1] . These processes form the scientific and engineering principles for water/wastewater treatment and hydraulic infrastructure. Nature-based solutions may be low cost, require low energy for operation and maintenance, generate low environmental impacts and provide added value through the benefits that accrue to humanity (ecosystem services). Ecoremediation achieves a high level of wastewater treatment and a water quality that meets the prescribed standard defined by legislation on wastewater discharge in Montenegro, as well as the standard defined by EU Directive 91/271/EEC (Didanovic, S., Sekulic, G., 2011, 2012) [2] - [11] .
The implementation of projects from the Ecoremediation Strategy in Montenegro has enabled a significant reduction in the costs of individual projects in relation to sector studies. Given the challenges of Montenegro in the field of wastewater, it can be concluded that ecoremediation in integrated form with existing strategies can successfully contribute to increasing environmental quality and reducing costs in the implementation of Council Directive 91/271/EEC of 21 May 1991 concerning urban wastewater treatment, which is planned in full by 2035, as well as significantly reducing the WWTP construction and maintenance costs [12] .
The following are some advantages and disadvantages of constructed wetlands, compared to conventional facilities (EPA 2003, Vrhovšek 2017, Malus 2012, Tushar, 2009, Vidali, 2001) [13] [14] [15] . Advantages of constructed wetlands (CW) are that they can be less expensive to build than other treatment options and that they utilize natural processes. They can be set up by simple construction (can be constructed with local materials), they necessitate simple operation and maintenance. In addition, they lead to cost-effectiveness (low construction and operation costs), and process stability. Limitations of constructed wetlands (CWs) are large area requirements. Also, wetland treatment may be economical relative to other options only where land is available and affordable. In addition, design criteria have yet to be developed for different types of wastewater and climates (UN-HABITAT, et al. 2008) [16] .
There are two basic types of constructed wetlands (Malus 2012, Tushar et al. 2009) [17] , which differ in the type of wastewater flow through them: constructed wetlands with surface wastewater flow and constructed wetlands with subsurface wastewater flow. In both types, it is extremely important to ensure the preliminary treatment of raw wastewater and the method of discharging wastewater and distribution in pools with vegetation (Malus 2012, Tushar 2009) [18] .
Constructed wetlands (CWs) consist of pools that are lined on one side with impermeable foil or clay, on which the substrate is placed (usually a mixture of sand and gravel, the ratio of which depends on the permeability of the substrate) in which selected plants are planted (Phragmites australis, Botur, Typha, Carex).
The use of CW often depends on the availability of the required construction area. The possibility of reducing the space required for constructed wetland (CW) has not yet been sufficiently explored. The impact of substrates on the efficacy of treatments is insufficiently examined, as is the impact on the required surface for CW area.
1.1. Constructed Wetland (CW) Sizing
There are many different guidelines for CW sizing summarized in a number of papers: Cooper (2005), Vymazal et al. (2008), Kadlec and Wallace (2009) et al. [19] [20] [21] . In determining the required area for CW, Brix and Johansen (2004) [22] define a simple rule according to which the constructed wetland A area (m2) is calculated as a triple multiple of the population equivalent with the sole aim of achieving 95% removal of BOD5 when using vertical subsurface flow constructed wetlands in a temperate climate zone. According to German guidelines (DWA, 2006) [23] , the required area of vertical subsurface flow constructed wetlands is calculated from the following formula: A (m2) = 4·PE (population equivalent), regardless of the influencing factors.
Danish guidelines Hans Brix, Carlos A. Arias (2005) [24] , the use of vertical flow constructed wetlands for on-site treatment of domestic wastewater: The necessary surface area of the filter bed is 3.2 m2/person equivalent and the effective filter depth is 1.0 m. The filter medium must be filter sand with a d10 between 0.25 and 1.2 mm, a d60 between 1 and 4 mm, and a uniformity coefficient (U = d60/d10) less than 3.5.
The vertical flow CW area varies, and the experiences of countries are different (Hrast T. 2012) [25] , so that an area of 1.53 m2 is required for the construction of these systems per PE in Italy, 5 m2 in Austria, 4.6 m2 in Denmark, and 2.3 m2 in Slovenia. The hydraulic load also varies, so it is 180 l/day PE in Italy, 150 l/day PE in Austria, 146 l/day PE in Denmark, and 145 l/day PE in Slovenia. The water retention time in these systems also varies.
For the sizing of wastewater treatment plants (Sekulić, 2015) [26] , it is important to keep in mind the following:
1) Population (expressed in PE)
2) Specific water consumption (usually calculated at 150 l/day per capita)
3) Daily amount of wastewater (Qd = Number of PE × 0.150 × 0.8)
The substrate depth for these systems also varies, so it is 1.1 m in Italy, 0.83 m in Austria, 1.1 m in Denmark, and 0.66 m in Slovenia. The slope of the bottom is 1.5% in Italy, 1.03% in Austria, 1.1% in Denmark and 1% in Slovenia.
The possibility of reducing the space required for constructed wetland (CW) construction has not yet been sufficiently explored. The impact of substrates on the efficacy of treatments is insufficiently examined, as is the impact on the required surface for CW area. In the previous practice of CW construction, multichambered septic tanks, Imhoff tanks or presetting tanks (Malus 2012, Tushar 2009) were mainly used in the phase of primary treatment, while other possibilities available on the market, such as newer generation extreme separators, use of efficient microorganisms, etc., have never been sufficiently investigated with the aim of reducing the potential space required for CW construction (in the secondary treatment phase).
1.2. Subject and Goal of Research
The subject matter of this research is to check the possibility of reducing the required area for CW construction (secondary treatment) by using an adequate substrate under the conditions in Montenegro.
The aim of this research is to examine the efficiency of municipal wastewater treatment under the conditions in Montenegro through the treatment in a CW vertical flow system on 3 different types of substrates used in Italy, Austria and Slovenia and thus the possibility of reducing the area required for CW construction, depending on the choice of substrate. Additionally, the aim of the research is to use the primary treatment of the existing WWTP in Podgorica to examine the effectiveness of secondary treatment in CW with 3 different substrate types.
The defined goals will be accompanied by setting up an experiment and applying methods for the analysis of physico-chemical parameters: t˚C, pH, TSS, COD and BOD5.
2. Materials and Method
2.1. Site Description
The pilot project CW was set up in the area of an existing WWTP in Podgorica (Figure 1). The existing WWTP in Podgorica is located in the settlement Krusevac, in the city centre, on the right bank of the Moraca River (Winsoft D.O.O. 2015) [27] . There is a wastewater treatment plant (WWTP) in Podgorica, which has been in operation since 1978. The site is within the existing WWTP in Podgorica in the city district of Krusevac.
WWTP is designed for a capacity of 55.000 PE and implements a biological secondary treatment with primary sedimentation and activated sludge process (DHV, December 2007) [28] .
2.2. Experimental Setup
In November 2020, the pool was set up, and a pilot project was constructed. The basic elements of the CW used in this experiment are shown in Figure 1. The constructed wetland (CW) has a vertical flow system and the role of secondary wastewater treatment in this experiment, receiving water from the existing WWTP in Podgorica after the primary treatment. The primary treatment at the existing WWTP is done for the purpose of removal of coarse material on coarse and fine screens, removal of inert material in aerated sand traps, and removal of sediment and suspended matter in primary sedimentation tanks. According to 2019 data obtained by municipal wastewater treatment plant management, regarding the incoming water or the influent, the percentage of COD and BOD5 of treated wastewater was reduced by approximately 31% after mechanical treatment (Source: archive of Podgorica Water and Sewage Corporation).
Scheme 1 shows the experimental setup in the settlement Krusevac-Podgorica city.
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Figure 1. Experimental setup, location of the existing WWTP in the settlement of Krusevac-Podgorica.
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Scheme 1. Layout of a vertical flow constructed wetland system (CW 1, CW 2 and CW 3) in the settlement Krusevac-Podgorica city.
2.3. Water and Air Distribution in the CW
After the primary treatment at the WWTP in Podgorica (Figure 2), wastewater is pumped by the pump (Villager VSP10000) into Pool 1 using a 1 m3 PVC water hose, and then through a PVC plastic pipe of DN 125 mm in diameter, it is pumped to the adjacent Pool 2 made of PVC with a volume of 1 m3, on which valves for water distribution are installed using PVC plastic pipes of DN 32 mm in diameter, and through plastic barrels with a volume of 60 liters, which have the role of water retention and additional sedimentation. Water from barrels I, II and III is distributed through PVC plastic pipes 32 mm in diameter into three different vertically constructed wetlands (CW-1, CW-2 and CW-3, made of PVC with a volume of 1 m3) filled with substrates of different granulation and water and air distribution pipes. These vertical CW fields represent the experiences of different countries in the application of secondary wastewater treatment using plants, such as Italy, Austria and Slovenia. In the CW surface zone, perforated pipes made of PVC plastic with a diameter of DN 32 mm are placed every 40 cm along its width, in addition to a side pipe through which water flows into the CW fields. These pipes (except the side one) are drilled every 10 cm (holes with a diameter of 6 mm) to enable the CW to be evenly soaked with wastewater. Inside the constructed wetlands (CW-1, CW-2 and CW-3), at the bottom, there are drainage pipes made of PVC plastic with a diameter of DN 75 mm that are drilled (notched 1/3 of the rim) every 20 cm to enable the reception of water passing through the substrate, and then the water is taken using a full pipe from the CW into the joint pipe (in the joint, this pipe is of DN 75 mm diameter) whose height later regulates the water level in the CW itself (Figure 1), and from there, after treatment, water is drained using PVC plastic pipes of DN 32 mm in diameter into a manhole located in the immediate vicinity.
Scheme 2 shows the cross section through constructed wetland (CW 1). The directions, diameters and types of the installed pipes are shown. The granulation, depth and structure of the substrate are also shown.
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Figure 2. Wastewater after primary treatment (PT).
2.4. Substrate Setting in CW and Plant Plants
In this experiment:
1) CW 1 vertical flow system for an area of 1 m2 represents the experience of Slovenia under the conditions in Montenegro (Scheme 2). The substrate depth for this system is 1.0 m. The filter medium is sand with a d10 between 8/16 mm, d60 between 0.5 and 4 mm, d10 between 4/8 mm, d5 between 8/16 mm, and d15 between 16/32 mm;
2) The CW 2 vertical flow system for an area of 1 m2 represents the experience of Austria under the conditions in Montenegro. The substrate depth for this system is 0.83 m. The filter medium is sand with a d5-10 between 8/16 mm, d50 between 0 and 4 mm, d5-10 between 4/8 mm or 8/16, and d20 between 8/16 mm or 16/32 mm.
3) The CW3 vertical flow system for an area of 1 m2 represents the experience of Italy under the conditions in Montenegro. The substrate depth for this system is 1.1 m. The filter medium is sand with a d20 between 16/32 mm, d60 between 0.4 and 8 mm, and d40 between 16/32 mm.In May 2021, after the construction and installation of the experiment, reeds were planted in all three troughs (CW1, CW2 and CW3) and transplanted from Skadar Lake, where they grow naturally.
2.5. Sample Collection and Analysis
Substrate efficiency analyses in the CW-1, CW-2 and CW-3 troughs were performed in the period of March-August 2021. Since the reed was rooted late, the efficacy of the substrate and the plants together will be examined in the following period because the plants need a certain period to develop a root system.
Analyzed parameters: biochemical oxygen demand (BOD5), chemical oxygen demand (COD), total suspended solids, temperature and pH. The analyses were performed in the laboratory located within the WWTP (Podgorica Water and Sewerage Corporation) at least twice a month in the period of March-August 2021. Wastewater sampling was performed at 6 points: 1) at the inlet, 2) after mechanical treatment, 3) from the pool to which water is pumped and where it is retained, 4) at the outlet after treatment in CW1, 5) at the outlet after treatment in CW2, and 6) at the outlet after treatment in CW3.
Wastewater dosing in the considered period was performed in several ways from 14 analyzed series, dosing was performed in such a way that a volume of 120 liters was dosed three times per day in each CW in three and four doses, a volume of 60 liters was dosed two times per day in one and three doses, a volume of 200 liters was dosed once a day in three doses, and a volume of 100 - 150 liters was dosed seven times per day in such a way that dosing was performed once, twice and three times per day.
3. Methods and Sampling Used
Sampling
Sampling is performed using an aluminum grip with a telescopic handle. The container on the gripper in which the sample is taken is plastic and has a volume of 1 l. Sampling was performed at 6 points: inlet water, water after primary treatment, pool, effluent after CW1, CW2 and CW3.
Temperature and pH
A mercury thermometer (PRECISION) with a scale division of 1/10˚C. is used to measure the temperature. The temperature was measured in a sample bottle with a volume of 1 L. The bottle must not be exposed to thermal or direct sunlight. The measurement is performed by placing the thermometer directly in the sample bottle, and the temperature is read only after a time period that provides constant values. Recording is performed at the nearest division of 0.5˚C. pH was measured with a pH meter WTW 315 (Gmbh D-82362 Weilheim).
Determination of chemical oxygen demand (COD) with potassium dichromate
In research standard methods for testing water quality (COD) were used, Coha (1990) [29] .
Determination of biochemical oxygen consumption after 5 days (BOD5)
In research standard methods for testing water quality (BOD5) were used, Coha (1990) and Lurje (1984) [29] [30] .
Procedure for determination of suspended substance (TSS) content
Standard methods for the examination of water and wastewater, 14th edition, 1975 [31] were used for determination TSS in the research.
4. Results
Table 1 shows the results of the analyzed samples in the period of March-August 2021. Wastewater analysis was performed at 6 points: at the inlet to the WWTP, after primary treatment, in Pool 1 (to which water is pumped after primary treatment), at the outlet (effluent) from CW 1 (substrate-experience of Slovenia), at the outlet (effluent) from CW 2 (substrate-experience of Austria), and at the outlet (effluent) from CW 3 (substrate-experience of Italy). There were 13 series of performed sample analyses, including 6 previously mentioned points, in total. The analyzed parameters were t (temperature), pH, suspended solids, chemical oxygen demand (COD), and biological oxygen demand (BOD5). Considering that the plants were rooted later (in the second half of June), as well as that they needed time to develop the root system, as concerns the incoming water, it can be concluded that the previous analyses mainly examined the substrate efficiency under weather conditions in Podgorica, Montenegro. Table 2 shows the percentage of SM, COD and BOD5 elimination after primary treatment for the incoming water, while Table 3 shows the percentage of SM, COD and BOD5 elimination after discharge from CW 1, CW 2, CW 3, with respect to the respective values for Pool 1 and the outgoing water.
Wastewater dosing in the considered period was performed in several ways; from 14 analyzed series, dosing was performed in such a way that a volume of 120 liters was dosed three times per day in each CW in three and four doses, a volume of 60 liters was dosed two times per day in one and three doses, a volume of 200 liters was dosed once a day in three doses, and a volume of 100 - 150 liters was dosed seven times per day in such a way that dosing was performed once, twice and three times per day.
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Table 1. Results of analyzed samples in the period march to august 2021.
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Table 2. Percentage of elimination of TSS, COD, BOD5 after Primary Treatment (PT).
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Table 3. Percentage of elimination of TSS, COD, BOD5 after CW1, CW2, CW3.
Treatment is intended to remove pollutants from wastewater to a certain extent (Sekulić, 2015) [26] . The level of wastewater treatment is defined by the formula (1):
(1)
RE—the percentage of removal of a particular substance from wastewater (%);
Cinf—concentration of a substance before treatment;
Cefl—concentration of a substance after treatment.
5. Discussion Results
The results indicate that the percentage of suspended matter (SM) elimination is as follows:
- in the CW1 effluent (experience of Slovenia), for the Pool and the influent, it averaged 57% and 69%, respectively;
- in the CW2 effluent (experience of Austria), for the Pool and the influent, it averaged 55% and 67%, respectively;
- in the CW3 effluent (experience of Italy), for Pool 1 and influent, it averaged 40% and 54%, respectively;
The results indicate that the percentage of COD elimination (Chemical Oxygen Demand) is as follows:
- in the CW1 effluent (experience of Slovenia), for Pool 1 and the influent, it averaged 51% and 63%, respectively;
- in the CW2 effluent (experience of Austria), for Pool 1 and the influent, it averaged 54% and 66%, respectively;
- in the CW3 effluent (experience of Italy), for Pool 1 and the influent, it averaged 56% and 70%, respectively;
The results indicate that the percentage of BOD5 elimination (Biochemical Oxygen Demand) is as follows:
- in the CW1 effluent (experience of Slovenia), for Pool 1 and the influent, it averaged 54% and 68%, respectively;
- in the CW2 effluent (experience of Austria), for Pool 1 and the influent, it averaged 57% and 70%, respectively;
- in the CW3 effluent (experience of Italy), for Pool 1 and the influent, it averaged 62% and 78%, respectively;
The results also indicate that the substrate efficiency in the CW2 trough (experience of Austria) in the first 11 series (before clogging) recorded the best results, where the percentage of SM elimination in the effluent for Pool 1 and the influent averaged 61% and 72%, respectively. The percentage of COB elimination in the CW2 effluent (experience of Austria) for Pool 1 and the influent averaged 61% and 70%, respectively, while the percentage of BOD5 elimination in the CW2 effluent (experience of Austria) for Pool 1 and the influent averaged 65% and 77%, respectively. The average results of the overall analyses indicate that in regard to the percentage of SM elimination, the best result was recorded in the CW1 effluent (for the Pool and the influent, 57% and 69%, respectively); it was approximate in CW 2 (for the Pool and the influent, 55% and 67%, respectively), while in CW 3, the percentage of SM elimination was significantly lower (for the Pool and the influent, 40% and 54%, respectively). The average results of the overall analyses indicate that in regard to the percentage of COD elimination, the best result was recorded in the CW3 effluent (for the Pool and the influent, 56% and 70%, respectively); it was approximate in CW 2 (for the Pool and the influent, 54% and 66%, respectively), while in CW 1, the percentage of HPK elimination was lower (for the Pool and the influent, 51% and 63%, respectively). The average results of the overall analyses indicate that in regard to the percentage of BOD5 elimination, the best result was in the CW3 effluent (for the Pool and the influent, 62% and 78%, respectively); it was approximate in CW 2 (for the Pool and the influent, 57% and 70%, respectively), while in CW 1, the percentage of SM elimination was lower (for the Pool and the influent, 54% and 68%, respectively).
The above indicates that depending on the choice of substrate, the efficiency of wastewater treatment can vary significantly, even in some cases by approximately 20%. Additionally, there is an evident influence of the quality of the effluent discharged after the secondary treatment in CW1, CW2, and CW3, depending on the efficiency of the treatment achieved in the primary treatment.
In the forthcoming period, it is expected that the impact of plants on CW1, CW2 and CW3 will be pronounced because plants have developed their root system, which will significantly affect the efficiency of wastewater treatment. Statistical significance analysis showed that there was no significant difference in performance between Phragmites australis and Scirpus regarding the removal of organic matter for a given organic load mass. The average removal efficiencies ranged between 62% (unplanted bed) and 70% (Scirpus) (Joana et al., 2010) [32] .