Bio-Char as an Adsorbent for Wastewater Purification

Abstract

This study was conducted to investigate the feasibility of use of olivepomace residues and citrus tree wood residues after burning at 550°(olivepomace charcoal and citrus charcoal) as alternative non-traditional substrates in vertical flow wetland systems (VFCWs) for removing inorganic pollutants and organic pathogens from municipal wastewater through secondary treatment for carbon emission reduction to combat climate change. The effectiveness of this treatment was examined by two pilot scale vertical flow constructed wetlands (VFCWs) systems with alternative substrates. Each system was designed from two operated units in series (two stages of treatment), each unit was manufactured with the same design and size. The difference between each system was the substrates used for treatment. The first system included olivepomace charcoal, while the second system included olivepomace charcoal with citrus charcoal. Both models were operated at the same time and under the same conditions. Both systems were tested with seven different hydraulic retention time (HRT) (12-24-36-48-96-144-192 hours). After conducting laboratory tests on wastewater samples after treatment for several biological, physical, and chemical tests, the results indicated that citrus charcoal and olive charcoal are effective alternative substrates in constructed wetland systems. The systemic way test results showed the lowest removal efficiency for TSS, BOD, COD, TP, TKN, and Fecal Coliform (95, 53, 44, 52, 40, and 66% at 12 hr), while the highest removal rates (97, 94, 94, 80, 69, and 98% at 12 hr), respectively in Model 1. And minimum removal rates for TSS, BOD, COD, TP, TKN and Fecal Coliform (95, 34, 22, 42, 48, and 50% at 12 hr), respectively, while the optimal removal (97, 98, 98, 71, 71, and 99% at 12 hr), respectively, in Model 2. This study proved that olive solid waste and tree wood residues are effective alternative substrates in removing pollutants from wastewater, which are inexpensive and environmentally friendly.

1. Introduction

Waste management is a global priority issue for the sustainability and preservation of natural resources and ecosystems. Recent studies are moving toward the management and treatment of organic food waste and their beneficial reuse, such as using them for the concurrent recovery of resource, water, and energy to facilitate civilizational sustainability and reduce environmental pollution to combat climate change [1].

In general, there are many environmental issues that need to be addressed and well managed to reduce the severity of environmental pollution, perhaps the most important of which is the issue of exacerbating the quantities of untreated wastewater, and these quantities increase with the increase in population numbers as a result of the increasing demand for various human activities such as drinking, domestic, agricultural, and industrial purposes. This leads to an increase in the amount of wastewater. Only 20% of wastewater overall gets satisfactory treatment and in low-income countries as low as 8% of the treatment capacity [2]. And the remainder is discharged without treatment or partially treatment, causing environmental damage and damage to public health. Every year, at least 1.8 million children under 5 years die from water-related diseases, according to a report published by the World Health Organization, due to unmanaged wastewater [3].

Moreover, there are increasing issues related to the aquatic environment, including water degradation, water shortage, and water pollution [4], and this issue has become a growing concern, in all countries, especially the countries of the Mediterranean region, which poses a threat to several sectors, the most important of which is the agricultural sector, which represents an important economic sector in many countries. Therefore, effective wastewater treatment and reuse for agricultural irrigation purposes can contribute to preserving potable water resources, overcoming the water shortage problem, and reducing the negative effects of wastewater associated with the release of liquid waste into the environment [5].

Wastewater treatment is both necessary and complex due to its complex composition [6]. Solids, organic matter, nitrogen, and phosphorus are general components of wastewater generated by urban communities [7, 8]. In addition, wastewater has pharmaceutical residues containing non-biodegradable toxic compounds and pathogenic microorganisms, which must be removed prior to final release to minimize environmental contamination and associated risks to human health [9, 10]. Historically, traditional centralized physico-chemical and biological technologies to sewage treatment have been used successfully for water pollution control in most countries [11, 12]. However, these wastewater treatment technologies such as membrane separation, membrane bioreactors, and activated sludge process are cost and not suitable for rural areas [13]. Furthermore, they are limited spread and insufficient when facing ever more stringent water and wastewater treatment standards [14]. Thus, selecting low-cost and efficient alternative technologies for wastewater treatment [15] is significant specially in developing regions [16].

Recently, the focus has been on developing systems and finding alternatives to reduce nutrient problems at its source of origin at low cost [17]. So the researchers have moved to using biological and physical remediation methods that are cost effective and cause no harm to the environment instead of chemical methods. Compared to conventional wastewater treatment technologies, constructed wetlands (CWs) are important alternative for decentralized wastewater treatment to solve water scarcity [18], in addition to its economical, environmentally friendly, and sustainable engineering systems due to their low cost, simple operation, and low maintenance [19, 20, 21, 22, 23]. CWs have used in worldwide purpose to improve water quality and reduce nutrients and pollutants in wastewater and maintain the diversity of the ecosystem [24, 25], without losing their ecological integrity or providing additional risks for global warming [4]. It is an energy efficient on-site technology that could be employed for wastewater polishing in urban areas with high population density due to small area requirement [26, 27].

This study combines the use of constructed wetlands as unconventional method and natural waste from solid waste as filler substrates instead of the usual substrates in constructed wetlands such as olive and plant wood waste to improve the efficiency of municipal wastewater treatment by integrating physical, chemical, and biological processes as an alternative to conventional treatment facility that requires high operating costs and energy.

Globally, olive oil industry is one of the most important industries in the agro-industrial sector in Mediterranean countries [28]. It is produced almost entirely in the Mediterranean region, especially in the State of Palestine, which covers most of its agricultural area with olive trees, approximately 57%, and relies mainly on industries based on olive oil [29]. Thus, the problem of environmental pollution from the waste of olive mills (OMW) increases in these producing regions, due to the increasing demand for olive oil all over the world.

Olive mills produce dark-colored wastes that contain liquid and solid wastes and a large amount of organic matter. It is composed of many complex materials that cannot be easily degraded. Production facilities are often associated with emissions of highly odorous volatile compounds. When their waste is dumped into the environment, it creates an odor nuisance, as it poses another threat to surface water and the environment when dumped without proper treatment and management, including changes in phytotoxicity, soil quality, natural water coloring, ground and surface water pollution that causes toxicity to aquatic animal life, and irritating odors, due to yeast treatment, paper, organic chemistry, winery, lignin, cellulose, hemicellulose, lipids, protein mill residues, and olive oil from olive pressing. It has a high chemical oxygen demand (COD >110 g/L), low pH and biological oxygen demand (BOD > 170 g/L), and a high organic toxicity load [28]. The main problem with olive mills waste disposal is to find an environmentally friendly and economically viable solution. In the biochemical treatment of wastewater from olive oil factories, capital and operating cost units must be installed with limited efficiencies due to high organic loads and chemical oxygen demand (COD) to biological oxygen demand (BOD) ratios. Since there are toxic organic materials that mostly come from the broken seeds, this waste is toxic to bacteria and direct biological treatment is not possible.

Therefore, olive oil-producing countries face a challenge to find feasible, economical, and sound ways to dispose of solid and liquid waste resulting from this industry. So, the solid waste from olive mills, especially the broken seeds in wastewater treatment, will be more useful and will add positive advantages to reduce the harmful effects and threats to the environment.

In this experiment, the wetland technique was used, built in an unconventional way, using natural substrates from the solid waste of olive presses. In this way, we can overcome several environmental problems in a smart, integrated, environmentally friendly way. Where the municipal wastewater is purified and treated using natural wastes from olive mills, and thus reducing the wastage of municipal wastewater, the problem of water scarcity will be overcome by providing an alternative source for irrigation and other purposes. Also, the quantities of waste from the olive mills will be disposed of properly instead of being exposed to the environment, which avoids damage to the soil, water, and air. Along with that, in this way built wetland technology is developed by the use of new natural substrates that are low in cost and operation. Thus, the desired goal of sustainability is achieved by simultaneously returning resources, water, and energy to reduce environmental pollution to combat climate change.

2. Materials and methods

2.1 Experimental setup

Two constructed wetland (VFCWs) models (namely model 1 and model 2) were built outdoors under prevailing environmental conditions, inside Sheikh Ajlin wastewater treatment plant (31°485,444′ N, 34°424,678′ E), Gaza, Palestine. This plant treats municipal wastewater using filtration basins as a first stage and aerobic, anaerobic treatment as a second stage. For the construction of the wetlands, work such as cleaning and extraction of the weeds from the site was initially carried out. Later, the excavation was executed, where drilling was carried out on several heights to put the wastewater tank and experiment basins consisting of two stages to transfer water by different levels through gravity without the need for operational energy.

2.2 Preliminary laboratory tests

Some analyses and preparations were carried out in the laboratory and on the field before the implementation of the experiment on the ground of Sheikh Ajleen Wastewater Treatment Plant, including: burning olivepomace and citrus plant wood waste at a temperature of 550° in isolation from oxygen, cracking citrus plant wood coal after burning to small sizes, substrate porosity calculation, sieve analysis for substrates, and substrate hydraulic calculation (sand, gravel, citrus plant wood coal, and olivepomace charcoal) to determine the suitability of the filter medium used in experiment models, where the size of the substrates was chosen from 1 mm to 40 mm with filter average porosity of 40%. The Phragmites australis plant was chosen to achieve the objective of the treatment and for growing them inside the Gaza Strip, 80 seedlings were transplanted and planted inside a basin at the Sheikh Ajleen plant before the wetlands were built and let to grow.

2.3 Constructed wetland setup

Two experimental models were built, each of which has two units (vertical flow sub-surface constructed wetlands). Each unit was made of strong wood covered on all sides with a layer of plastic liners mad of PVC with a thickness of 2.0 mm to prevent any leaks down, and this type of liner is available in local markets and its price is acceptable, which also has high resistance to the chemicals found in wastewater, and it is characterized by high flexibility when there are seasonal changes such as temperature changes [30]. All units had the same volume = 0.80 m × 0.40 m × 0.70 m, aspect ratio (2:1) with 45% average porosity, calculated in the laboratory using liquid saturating method and 1000–10,000 m²/m²/d hydraulic conductivity, calculated in the laboratory using constant head method. Generally, the porosity should be used ranges from (0.4:1) to (3:1), which ensures high porosity and a high hydraulic conductivity to provide even influent liquid distribution and efficient infiltration, and facilitate and improve plant growth and collection of influent liquid.

In this experiment, the Phragmites australis plant was used in the eight basins. Their roots extend to depth of 0.7 m [31]. (Accordingly, the depth of each basin was 0.7 m, to take into account the growth of roots well and the occurrence of biological interactions between the liquid and the root.) In CWs, it is important achieve a slope in the lower layers of the bed to achieve a horizontal flow along the bed [32]. In general, the preferable hydraulic gradient S < 10% of maximum potential gives the adaptability and the reserve capacity for future operational changes [30, 31]. For all basins in this experiment, the bed slope is designed with 5% slope as shown in Table 1.

Parameter	Value	Unit
Total depth	0.70	$\mathrm{m}$
Long-wide (aspect ratio)	2:1
Long	0.80	m
Width	0.40	m
Surface area	0.32	m^2
Total volume	0.224	m^3
Slope	5	$\%$
Depth of filter medium	0.60	m
Filter average porosity	0.45	$\%$
Water depth in filter medium	0.55	m

Table 1.

Data of the design parameter to vertical constructed wetlands.

The basins were placed with a slope to facilitate the flow of wastewater whereas transport pipes of 50 mm diameter were installed from the wastewater feeding tank to the other upper four VFSCWs. In each unit, two plastic vertical pipes were installed, 1 pipe used for every 4 inch to increase the amount of oxygen in the wetlands [33].

Experimental models were arranged to form two parallel model trains: models 1 and 2. Each model included VF sub-surface constructed wetland unit as the first stage (A), it represents the first stage of treatment, and another VF sub-surface constructed wetland unit as the second (B) stage, it represents the last stage of treatment, as shown in Figure 1.

Figure 1.

Wetlands built to scale. (a) Coarse gravel, (b) medium gravel, (c) fine gravel, (d) loam sand, (e) citrus charcoal (f) olive pomace charcoal, (g) Phragmites australis.

The aim of this study was remove organic pollutants presented in post-treated wastewater by using CWs, so the kinetic model was used to estimate the concentration of BOD(Co) after treatment based on the general form of Eq. (1) [34].

$$\frac{C_{\mathit{e}}}{C_{\mathit{o}}}={\mathit{e}}^{\left(-\mathit{KTt}\right)}$$E1

Where,

${\mathrm{C}}_e$ = effluent BOD5 (mg/l), ${\mathrm{C}}_o$ = influent BOD5 (mg/l), K_T = temperature dependent rate constant ($d^{-1}$), t = hydraulic retention time (d).

$${\mathrm{K}}_{\mathrm{T}}={\mathrm{K}}_{20}{\left(\mathit{\theta }\right)}^{\left(\mathit{T}-20°\right)}$$E2

Where,

K₂₀ = temperature coefficient for rate constant, T = temperature of liquid in the wetland (°C).

K₂₀ equals 1.104 (d-1), $\theta$ =Arrhenius coefficients used were 1.06 for BOD.

To calculate the surface area required the first-order BOD removal equation was used proposed by Kickuth [34], based on Eq. (3).

$$\mathit{\text{As}}=\mathit{\text{(L)}}\mathit{\text{(W)}}\frac{\mathit{Qd}[ln\left(\mathrm{Co}/\mathrm{Ce}]\right)}{\mathit{\text{KBOD}}}$$E3

Where,

Αh = Surface area of bed (m²), Qd = average daily flow rate of sewage (m³/d), KBOD = rate constant (m/d), KBOD is determined from the expression Ktdn, where, KT = K20 (1.06) (T-20°), K20 = rate constant at 20°C (d-1), d = depth of water column (m), n = porosity of the substrate medium (percentage expressed as fraction).

Based upon Eq. (3), the required surface area (As) was 0.32 m² assuming the aspect ratio (L:W) is 2:1, so the length is 0.8 m and width is 0.4 m, the depth of the filtration media is 0.6 m, while the depth of water column is 0.55 m to maintain a level of 0.05 m under the filtration media as mentioned previously. Therefore, the total volume arithmetic equal is 0.224 m³, and the total net volume arithmetic is 0.176 m³, for each basin.

2.3.1 Filtration media

The types and sizes of the substrates were carefully chosen in this experiment to achieve the greatest degree of removal. Sandy loam, gravel, and coal were used, away from using the clay due to the low hydraulic conductivity, and working on the recommendations of previous studies such as unifying the size of the substrates, entry and exit areas using large filter media to prevent blockage, and accumulation ranging between 40 and 80 mm [35]. On the contrary, in the upper layer, a large size of media is not preferred used as it is not suitable for plant root propagation, but for the vegetable layer, it is preferred to use sand with gravel to filter and facilitate the plant growth process [36], and use gravel at the inlet and outlet to help prevent blockage [36]. This experimental basins were packed with biological and construction materials that are gravel, sand, citrus charcoal, and olive pomace charcoal. Module 1 is the same as the module 2, but was replaced by the fourth layer with olive pomace charcoal substrate in module 1, and mix olivepomace charcoal and citrus charcoal substrate in module 2.

Coarse gravels (30–40 mm) were used as supporting layer at the bottom of basins, of 15 cm thick, as shown in Figure 1(a), and this size was chosen because there was a valve in the bottom of each basin, which was used for controlling the flow in the module, so it was chosen larger than the entrance of valve (which was 1/2 inch), so the valve would not be clogged by small aggregates and facilitate effluent discharge. The second layer was 7.5 cm thick and filled with medium gravel (20 mm), as shown in Figure 1(b), the third layer was 7.5 cm thick and filled with fine gravel (10 mm), as shown in Figure 1(c), the fourth layer was 15 cm thick filled with olive pomace charcoal (1–19 mm) in the first model, as shown in Figure 1(f); the second module used a mixture layer of citrus wood waste charcoal and olive pomace charcoal in equal proportions (1–19 mm), the fifth layer was 15 cm thick having a mixture of loam sand and small gravel at the ratio of 1:3 (aggregates: loam sand, respectively) as shown in Figure 1(d), to support the growth of plants, and the upper part was empty of about 10 cm to allow aeration for the module and the roots of the plants. The common Phragmites australis plant was planted in all basins for the two models. Its roots extend to a depth of 0.7 m [4], so the depth of the substrate (0.7 m) was studied taking into consideration the greater root depth of the Phragmites australis plant. After planting the Phragmites australis plants in basins, they were fed with water for 20 weeks for growth and maturation. Substrates including sand, gravels, olivepomace charcoal, and citrus wood waste charcoal firstly were washed with tap water to remove ashes and then filled into the experience basins to a height of 60 cm. And in general each basin had an average porosity of 40% with an average working volume of 75 L, which is high porosity and a high hydraulic conductivity to provide efficient infiltration.

2.3.2 Inlet structure and outlet structure

Inlet structures play an important role in treatment wetlands by aiding effective flow distribution across the full width of wetland bed. In the four experimental basins, the main pipe of the feeding tank was used, which was 3 m length and 32 mm diameter. And four separate inlet pipes for each basin with the same length and diameter of 20 mm and 80 cm length were placed horizontally above the basins to allow the liquid to descend down through the unit layers evenly, each open pipe with equal openings and distance from each other at an equal distance to achieve distribution in all basin directions evenly and each pipe connected with a valve for open and close and controlling the flow liquid speed. While the function of the outlet in wetlands is to collect the effluent liquid after treatment toward the reuse of places or storage, there are several types, sizes, and location selected depending on the design [4].

In this experiment, four outlet pipes of the same shape and size were used with valve for each of them to close and open. The two inlet pipes of the second stage are the same as the outlets for the two basins of the first stage, the other four outlets were an exit for the second stage connecting with two pipes having each of length 2 m and 20 mm diameter with valve; and it was buried under the sand at distance of 2 m away from the module to discharge the wastewater after its treatment to the agricultural area inside the treatment plant.

2.4 Wastewater feeding and flow pattern

The CWs were feed with one type of wastewater pumped from the Gaza City wastewater treatment plant (municipal wastewater secondary treatment using aeration basins) for a total period of 10 months. The first 6 months allowed system stabilization; experimental analyses were conducted within the remaining 4 months.

All CWs in this study were designed as vertical down-flow subsurface flow constructed wetlands, each module had of two units (i.e., two-stage treatment) arranged on top of each other, and the layers of each unit arranged for vertical flow in the bed without a pump. The influent flows from inlet (wastewater feeding tank) toward the first two upper basins (i.e., first treatment stage) and then flows through the porous media under the surface of the beds in a vertical path via gravity (down-flow) until it reaches the outlet of second two lower basins (i.e., second stage and final treatment stage) to complete the treatment process, where it is collected. The flow was continuous at all stages for all trains in parallel, and the flow quantity was controlled using manual valves for each basin.

2.5 Process conditions

2.5.1 Hydraulic loading rate

To achieve the high removal efficiency for raw sweater within constructed wetlands, this requires long hydraulic retention with low hydraulic loading rate [37].

In this experiment, the hydraulic loading rate was determined based on the dimensions of the basins, where the design was for all basins similarly, according to the following Eq. (4):

$$\mathrm{HLR}=\mathrm{Q}\backslash \mathrm{A}$$E4

Where,

HLR: hydraulic loading rate (m/d), Q: flow (m³/d), A: surface area of the constructed wetland (m²).

As mentioned earlier, more than one hydraulic retention time (HRT) was used to test the efficiency of the best, so the hydraulic loading rates were according to the HRT as shown in Table 2.

HRT (h)	Flow (m^3/d)	Hydraulic loading rate (m^3/m^2/day)
12	0.150	0.47
24	0.075	0.23
36	0.050	0.16
48	0.038	0.12
96	0.019	0.06
144	0.013	0.04
192	0.009	0.03

Table 2.

Flow rate and hydraulic loading rate for each basin at seven HRTs.

2.5.2 Hydraulic retention time

The mean actual HRT is the average time that the water remains in the CWs. One of the objectives of this study is to evaluate the effect of HRT on CWs contaminant removal efficiency under the same conditions. So seven HRTs are assumed and the samples are collected for each of hydraulic retention time (HRT). These were in the order of HRTs at 12, 24, 36, 48, 96, 144, and 192 h, to study the relationship between retention time and treatment efficiency. Between each HRT, the modules rested for 2 days to allow re-oxygenation of the substrate.

2.6 Sampling and analyses

Wastewater was collected from inlet and outlet across VF wetland models at different hydraulic retention times (HRTs) (12, 24, 36, 48, 144, and 192 h). The HRT was controlled using a digital timer regulator. The effect of hydraulic retention time and substrate bed was studied to find out the best system efficiency with HRT.

The temperature of the treated wastewater was (28 ± 2)°C during the study. The wastewater samples of VSSF – VSSF two systems, including influent and effluent from each system, were taken three times at HRT to monitor the system performance. In each analysis, three samples, two influent samples and one effluent sample, were collected separately.

The total samples for the influent and the effluent were 21 and 21 samples, from each system, respectively, and at each sampling time, triplicate samples were collected for analysis.

The constructed wetland sample units were collected in sterile plastic bottles. The volume sampled was 1 L for each sampling point. The sampling recipients were 1-L plastic bottles for standard parameters and sterilized 50-mL glass bottles for Fecal Coliforms. It was collected in glass bottles, and then mixed to form a composite sample and stored at 4°C. The most important challenge that was at the beginning of the experiment was to collect the substrates that were used, burned, and cracked to the required sizes when collecting samples manually.

Collected samples were analyzed immediately at the laboratory of Institute of Water and Environment and Chemistry Laboratory at Al-Azhar University Gaza and Department of Environmental and Sciences at the Islamic University Gaza.

The analyzed parameters included: PH, turbidity, electrical conductivity (EC), total dissolved solid (TDS), total suspended solid (TSS), dissolved oxygen (DO), ammonium nitrogen (NH4-N), phosphate (PO₄), nitrate nitrogen (NO₃-N), total nitrogen (TN), total phosphorus (TP), ammonia (NH₃), 5 days biochemical oxygen demand (BOD5), chemical oxygen demand (COD), chloride (Cl), calcium (Ca), magnesium (Mg), hardness, phosphate (PO₄), sodium (Na), potassium (K), total Kjeldahl nitrogen (TKN) and Fecal Coliform. The testing methodology was done according to the standard methods stated in the Examination of Water and Wastewater book edited by ANDREW D. EATON, LENORE S. CLESCERI, ARNOLD E GREENBERG, 1995 [38].

3. Results and discussion

3.1 Total suspended solid (TSS) removal

The total suspended solids (TSS) are the dry weight of suspended particles that are not dissolved, in a sample of water that can be trapped by a filter that is analyzed using a filtration apparatus. Water quality parameter is used to assess the quality of a specimen of any type of water or water body, ocean water for example, or wastewater after treatment. TSS includes the organic matter (volatile) and inorganic matter (fixed) [39]. There are several mechanisms to remove suspended solids in the water including flocculation, interception, adhesion, straining, impaction, and filtration [40]. This test is a good measure of microbial contamination and turbidity to assess wastewater quality after treatment [41].

Table 3 shows the measured influent and effluent concentrations and removal efficiency for TSS. The concentration of suspended materials from the influent was few, due to feeding of the constructed wetland modules from the secondary treatment unit, where the wastewater underwent primary and secondary treatments (secondary treatment of municipal wastewater). After entering into constructed wetlands for treatment there was a noticeable decrease in concentration since the first hours in all modules. The removal rates in both systems were almost close to each other. The highest removal percentage of TSS for both systems was 97 and 96%, respectively, at 192 HRTs, and the lowest removal percentage was 95 and 95%, respectively, at 12 h as shown in Figure 2.

	HRT (hr)	Average influent (mg\l)	Load In ${\mathbf{gm}}^{\mathit{-}\mathbf{2}}$${\mathbf{d}}^{\mathit{-}\mathbf{1}}$	Average effluent (mg\l)		Removal efficiency (%)		Load out g${\mathbf{gm}}^{\mathit{-}\mathbf{2}}$${\mathbf{d}}^{\mathit{-}\mathbf{1}}$
Parameter				Model (1)	Model (2)	Model (1)	Model (2)	Model (1)	Model (2)
TSS	12	3.9	0.90	0.21	0.21	95	95	0.049	0.049
	24	3.9	0.45	0.17	0.2	96	95	0.020	0.023
	36	3.9	0.30	0.17	0.18	96	95	0.013	0.014
	48	4.3	0.26	0.16	0.2	96	95	0.010	0.012
	96	4.3	0.13	0.16	0.2	96	95	0.005	0.006
	144	4.5	0.09	0.15	0.2	97	96	0.003	0.004
	192	4.5		0.15	0.2	97	97	0.002	0.003

Table 3.

Measured TSS influent and effluent concentrations and input-output loading rate values for the (2 stages VF-CWs) module replicated seven times.

Figure 2.

Measured TSS influent and effluent concentrations during the operational period.

Mechanism of pollutant removal may change as a function depending on the concentration of pollutants entering, type of substrates, type of vegetation used, and wetland hydrology.

Physical, chemical, and biological factors such as: sedimentation, filtration, interception, settling, adsorption, and sagging all affect the bed layers of the systems and form the basis for the removal of suspended solids [42].

In this study, there was an effective removal of suspended solids in all modules. One of the essential mechanisms in the removal of suspended solids in these modules is the sedimentation and filtration of particulates, due to the arrangement of the layers, the type of substrates, and the flow velocity used in this experiment, this is consistent with the studies [43, 44].

Also, plants played a role in the removal of TSS in all modules. Where all the modules were planted along Phragmites australis plant, this plant needs temperature and a sufficient amount of oxygen to carry out the photosynthesis process for good growth and growth of microorganisms. In subsequent steps, the organic matter suspended solids are removed in the wastewater [45] and refractory compounds [46]. As the network of bacteria formed around the roots and on substrate surfaces within the bed, it acts as a strong filtration agent for removal [42]. Another factor that affected the removal of TSS was hydraulic ration time, it was clear that with increasing contact time the removal of the both s modules increased, these results are consistent with it [42].

Generally, the results of this study were more efficient than previous study [47], which used inorganic types of substrates such as gravel and sand; this explains that olive pomace charcoal and citrus charcoal have a higher efficiency than gravel for removal of TSS, perhaps the reason for the increase in the surface of the substrate on the coal seams compared to the surfaces of inorganic materials, especially in the first model that contained one type of substrate, which gives more homogeneity that increases the surface. This means the type of substrate plays an important role in TSS removal, and these results are consistent with it [48].

It was noticed that the input loading values of TSS were decreasing with increase of HRT (Figure 3). However, the graph shows that all modules have the same behavior, there was continuous eliminated values of TSS with increasing HRT, the first model recorded the lowest elimination values, while the second module recorded the highest TSS elimination values at all HRTs, as for the third and fourth models, they were close to the results of the second models during the operating period.

Figure 3.

The input-output loading rate values of TSS through the operational period of the system replicated seven times.

3.2 Biological oxygen demand (BOD) removal

Biological oxygen demand (BOD) or another common term is biological oxygen requirement [49]. It measures quantity of oxygen required for microorganisms to oxidize or degrade organic waste present in water. The BOD test is also known as “BOD5” that measures dissolved oxygen during a five-day incubation in dark conditions at 20°C. This test is useful for measuring water and wastewater quality, the unit of measurement mg/l [50].

In this study, average influent concentration during the operation period was 249 mg/l. The results presented in Table 4 revealed that the constructed wetland noticeably improved the effluent quality of BOD for all models.

	HRT (hr)	Average influent (mg\l)	Load in ${\mathbf{gm}}^{\mathit{-}\mathbf{2}}$${\mathbf{d}}^{\mathit{-}\mathbf{1}}$mg\l	Average effluent (mg\l)		Removal efficiency (%)		Load out g ${\mathbf{gm}}^{\mathit{-}\mathbf{2}}$${\mathbf{d}}^{\mathit{-}\mathbf{1}}$
Parameter				Model (1)	Model (2)	Model (1)	Model (2)	Model (2)	Model (2)
BOD	12	275	48.9	131	182	53	34	30.6	42.7
	24	275	21.2	110	125	60	55	12.9	14.7
	36	275	10.2	65	113	76	59	5.1	8.8
	48	220	3.6	28	75	87	66	1.7	4.5
	96	220	0.6	24	27.5	89	88	0.7	0.8
	144	240	0.4	16	14.6	94	94	0.3	0.3
	192	240	0.3	15	5	94	98	0.2	0.07

Table 4.

Measured BOD influent and effluent concentrations a for the (two-stage VF-CWs) module that replicated seven times.

Where the highest effluent BOD concentration value after the treatment was 131 mg/l in the first model at 12 h, the lowest concentration value was 5 mg/l at 192 h in the second module, as shown in Figure 4.

Figure 4.

Measured TSS influent and effluent concentrations during the operational period.

The lowest removal rates of BOD were 53 and 34%, respectively, at 12 h, while the highest BOD removal rates were 94 and 98%, respectively, at 192 h. In the both modules, the process of removing pollutants continued increasing with increasing contact time, which gave varying results.

The availability of oxygen in wetlands is the main factor for the decomposition of organic matter in wastewater, where the availability of good porosity in addition to the roots of plants promotes the formation of micro-aerobic and anaerobic environments and thus accelerates the development of microbial communities [51].

The main source of ventilation was in the experimental models due to the high percentage of voids within the layers that are transported by the roots of plants. Where the average porosity was 40%, in all experimental models, all were planted with common reed plants, which promoted decomposition and removal of organic matter.

All models gave high results for removal of BOD, but at different rates. The removal process in all modules was effective with increasing the HRT. Perhaps the reason for the type of substrates used in both models is these results are consistent with it [48].

The curve shows the eliminated values of BOD for four repeated experiments using different types of substrates (Figure 5), where the input loading values were decreasing with increase of HRT. However, the graph shows that all modules have the same behavior, and there was continuous eliminated values of BOD with increasing hydraulic retention time. Where the second model achieved the highest eliminated values of BOD at 12 h, while the fourth model, which gave the least eliminated values at 12 h, then it achieved the highest eliminated values at 192 h. As for the second and third modules, it had almost the same behavior.

Figure 5.

The input-output loading rate values of TSS through the operational period of the system replicated seven times.

3.3 Chemical oxygen demand (COD) removal

Chemical oxygen demand (COD) is a measure quantity for the amount of oxygen required to oxidize particulate organic matter that is soluble in water [4].

The influent average concentration during the operating period was between 415 and 437 mg/l. Where the highest concentration effluent of COD value was 245 mg\l in the first system at 12 h, the lowest COD value was 10 mg/l at 192 h in the second system, as shown in Table 5.

	HRT (hr)	Average influent (mg\l)	Average effluent (mg\l)		Removal efficiency (%)
Parameter			Model (1)	Model (2)	Model (1)	Model (2)
COD	12	437	245	340	44	22
	24	437	205	232	53	50
	36	437	123	211	72	52
	48	415	51	140	88	66
	96	415	44	52	89	88
	144	430	29	26	93	94
	192	430	28	10	94	98

Table 5.

Measured COD influent and effluent concentrations a for the two-stage VF-CWs module replicated seven times.

In both models, continuous removal of COD was observed, but the second system achieved slightly higher efficiency than the first system after 96 h, on subsequent increase in the hydraulic retention time, as shown in Figure 6. The minimum removal rates in systems 1 and 2 were 44 and 22%, respectively, at 12 HRTs, while the maximum removal rate for both systems reached 94 and 98%, respectively, at 192 HRTs.

Figure 6.

Measured TSS influent and effluent concentrations during the operational period.

The reason for that may be due to the size of the heterogeneous substrates and the interaction between olivepomace charcoal and citrus charcoal. Oxygen-consuming conditions encouraged the development of aerobic microorganisms and helped the degradation of organic matters [52].

Based on the previous results, it is clear that there is a relationship between the biological oxygen demand and chemical oxygen demand represented by the following equation: COD ≈1.8 BOD.

This type of wetland with organic materials gave higher removal efficiency of COD and BOD compared to another study, which used organic and inorganic materials as substrates: sugarcane bagasse, biochar, coal, and oyster shell; rushed mortar, recycled bricks, gravel, and sand [22].

3.4 Total phosphorus (TP) removal

Table 6 shows the removal percentages of TP at different hydraulic retention times. Generally, phosphorus in CWs is removed by plant uptake, microbial immobilization, substrate adsorption and accretions of wetland soils.

	HRT (hr)	Average influent (mg\l)	Average effluent (mg\l)		Removal efficiency (%)
Parameter			Model (1)	Model (2)	Model (1)	Model (2)
TP	12	13	6.3	7.6	52	42
	24	13	6.6	6	49	54
	36	13	5	5.5	62	58
	48	14.7	4.3	5.7	71	61
	96	14.7	3	4.2	80	71
	144	13	2.8	4	79	70
	192	13	2.6	3.8	80	71

Table 6.

Measured TP influent and effluent concentrations a for the two-stage VF-CWs module replicated seven times.

In this experiment, the removal rate was nearly halved in both regimens after 12 h (adsorption period) and the removal continued to increase when the HRT increases. Where the minimum removal rates for models 1 and 2 were 52 and 42%, respectively, at 12 HRTs and the maximum removal rate of both systems reached 80 and 71%, respectively, at 192 HRTs, as shown in Figure 7. This indicates that the HRT is a crucial parameter for the effective P removal [53]. These results are more efficient than previous study [54], which used inorganic materials such as maifanite, steel slag, bamboo charcoal, and limestone. This indicates that olivepomace charcoal substrates are suitable for removing phosphorous, and it is best to use as a layer alone without mixing with other substrates to give higher efficiency.

Figure 7.

Measured TP influent and effluent concentrations during the operational period.

3.5 Total Kjeldahl nitrogen (TKN) removal

The total Kjeldahl nitrogen (TKN) is defined as the sum of ammonia (NH₄ and NH₃) and organic nitrogen (Org-N) [55]. In wastewater, the presence of organic nitrogen is due to deamination reactions during the metabolism of organic matter [56].

High TKN concentrations were detected in the influent throughout the experimental during the operating period; the average influent concentration was 216 mg/l.

Table 7 shows the results of total Kjeldahl nitrogen (TKN) analyses for all models during the operating period. It is clear that the lowest removal rates for the both models were 40 and 48%, respectively, at 12 of HRT, while the highest removal rates for the modules were 69 and 70%, respectively, at 192 of HRT. The results showed that the TKN removal rates gradually increased with increasing contact time for all models without exception.

	HRT (hr)	Average influent (mg\l)	Load in g ${\mathbf{gm}}^{\mathit{-}\mathbf{2}}$${\mathbf{d}}^{\mathit{-}\mathbf{1}}$; g${\mathbf{m}}^{\mathit{-}\mathbf{2}}$${\mathbf{d}}^{\mathit{-}\mathbf{1}}$	Average effluent (mg\l)		Removal efficiency (%)		Load out g ${\mathbf{gm}}^{\mathit{-}\mathbf{2}}$${\mathbf{d}}^{\mathit{-}\mathbf{1}}$
Parameter				Model (1)	Model (2)	Model (1)	Model (2)	Model (1)	Model (2)
TKN	12	230	53.9	137	120	40	48	32.1	28.1
	24	230	26.9	120	109	48	53	14.1	12.8
	36	230	17.9	112	93	51	60	8.8	7.3
	48	200	11.9	96	74	52	63	5.7	4.4
	96	200	5.9	84	66	58	67	2.5	1.9
	144	210	4.3	78	64	63	70	1.6	1.3
	192	210	2.9	65	62	69	71	0.9	0.8

Table 7.

Measured TKN influent and effluent concentrations and input-output loading rate values for the two-stages VF-CWs module replicated seven times.

The minimum value were in model 2 (concentration 62 mg/l - removal efficiency 71% at HRT 192 h) and the maximum value were in model 1 (concentration 137 mg/l - removal efficiency 40% at HRT 12 h) as shown in Figure 8. Hence, it is possible to notice that, HRT and material are factors that possibly influence TKN removal in CWs.

Figure 8.

Measured TKN influent and effluent concentrations during the operational period.

In general, the process of removing nitrogenous compounds depends on the aerobic and anaerobic conditions provided by layers and plant roots [57]. Where the plant contains biomass in the lower area of “stems and roots” and the upper region “leaves”, where it can be grown either subsurface horizontally or vertically and create enormous networks with the bed particles and soil; and thus form a suitable surface to absorb nutrients and ions [45, 47]. Also, increased nitrification and denitrification result from plant respiration provides aerobic conditions and develops microorganisms at the root [58].

The removal rates in wetlands vary from plant to other, wherein the study [59], four types of plants were used to remove nutrients: Typhalatifolia, Phragmites australis, Scirpus (Bulrush), and Alismaplantago, the results indicated that the systems constructed with Typhalatifolia, Phragmites australis plant, which gave a higher efficiency in removing nitrogen and phosphorous compared to other plants used. In this experiment, Phragmites australis plant was used, as the results showed that almost half of concentrations were removal in all modules. To obtain better removal rates, continuous ventilation unit must be provided with the plant in modules, in this experiment, the porosity of the layers was the main factor for ventilation without the presence of a continuous ventilation unit, so the lack of an essential continuous source of aeration in the modules gave these results because the limited oxygen reduces the removal efficiency, especially in the early stages of operation. With the passage of time and plant growth, the efficiency of TKN removal will improve [60].

Other factors contributed to the removal of TKN, including the temperature. The temperature of wastewater in this experiment was moderate between 27 and 30°C, as the temperature affects the rate of nitrification, which is able to promote plant growth, release of root secretions, and microbial activity [56]. Thus, the rate of nutrient uptake and the rate of denitrification are accelerated, as nitrification reaches a maximum rate at temperatures between 30 and 35°C [61]. Also, the organic materials used played a role in the removal of TKN, where the presence of ammonia and organic carbon leads to the adsorption of microorganisms and the decomposition of the pollutants in addition to changing oxidation conditions due to the diversity and abundance of the microbial community. As it helps remaining carbon source, it can be used as an electron donor in the denitrification process. These results are consistent with the study [48].

In addition, in all models there was an increase in removal with increase of HRT, as shown in Figure 9. This corresponds to some studies that indicated HRT is one of the factors affecting nitrogen removal. Moreover, the author studies found a decrease tendency on the TKN removal during 1 year and a half of operation, this is a consequence of the vegetative cycle of the plants, since their absorption capacity decreases as the plants age [58]. It was observed in this experiment that there was a slight discrepancy in the removal rates between the first and second systems, where the second system, which contained organic substrates mix of olivepomace and citrus charcoal, gave slightly higher results than the first system that contained citrus charcoal only. The explanation may be due to interaction between the different substrates.

Figure 9.

The input-output loading rate value of TSS through the operational period of the system replicated seven times.

3.6 Fecal Coliform (CFU/100 ml) removal

A pathogen in the oldest and broadest sense is any organism that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ. Typically, the term is used to describe an infectious microorganism or agent, such as a virus, bacterium, protozoan, prion, viroid, or fungus humans [62]. The presence of pathogens such as bacteria and viruses in treated wastewater is of concern to public health and the environment if not disposed it. Pathogens are associated with many human diseases when a person comes into contact with water contaminated with bacteria, viruses, and other pathogens [4]. These pathogens are natural factors everywhere including humans, animals, and wastewater. Wastewater is the main source of these pathogens and an important source of transport to humans [63]. Where Fecal Coliform are considered one of the serious pathogens in wastewater and their presence as a function of pollution [60]. Previous studies have shown that hybrid or integrated systems have a higher efficiency in removing pathogens from single-phase systems. Some have suggested that wetlands be combined with chemicals or physical substances to improve performance so that when effluent is formed treatment is suitable for irrigation. In general, wetlands have proven their ability to remove pathogens significantly is well recognized and that pathogen removal in CW is greatly influenced by several factors including: temperature sunlight/UV radiation, pH, hydraulic loading, retention time, texture and porosity, water depth, flow rates, vegetation, and substrate media [63]. Some studies have shown that there is no effect of changing seasons on the process of removing pathogens, while other studies have shown higher removal of pathogens in warmer season and in the dry season in equatorial regions [63]. In this experiment, there was effective removal of Fecal Coliform in both models at (28 ± 2)°C, which means suitable conditions for removal. In addition, the removal of pathogens in CWs is greatly influenced by hydraulic retention time (HRT) [64]. This is in agreement with the results of this experiment, the models achieved an increased removal of Fecal Coliform with an increase of HRT.

In this experiment, there was effective removal of Fecal Coliform in both systems, which means suitable conditions for removal. The results of the experiment achieved high removal with increased HRT for all models. The highest level removal of Fecal Coliform reaches around (99%, module 2–192 hr), from 81,264 to 700 Cfu/100 ml, while the lowest removal level of Fecal Coliform reaches around (16%, module1–12 hr) from 81,264 to 67,980 Cfu/100 ml. The highest removal rates were as follows: (98, 99%, respectively) after 192 hours, while the lowest removal rates are as follows (66, 50%, respectively) after 12 hours for the bath modules, as shown in Table 8.

	HRT (hr)	Average influent (CFU/100 ml)	Average effluent (CFU/100 ml)		Removal efficiency (%)
Parameter			Model (1)	Model (2)	Model (1)	Model (2)
Fecal Coliform	12	81,264	28,000	40,900	66	50
	24	81,264	14,500	20,000	82	70
	36	81,264	10,000	11,900	88	85
	48	81,264	8150	4550	90	94
	96	81,264	4184	2200	95	97
	144	81,264	2000	1000	98	99
	192	81,264	1500	700	98	99

Table 8.

Measured fecal coliform influent and effluent concentrations and input-output loading rates values for the two-stages VF-CWs module replicated seven times.

The results also gave clear differences and higher efficiency in removal compared to other studies that used types of gravel and inorganic substrates, and this is due to olive residues and organic plant wood residues that were used in this study. This explains the role of substrates used in the treatment, especially module 1 that may be for the homogeneity of the material and this is consistent with [64]. In general, this wetlands have proven their ability to remove pathogens significantly and treated water is suitable compared to the Palestinian standards for the re-use of treated wastewater (Ministry of Environmental Affairs) (Figure 10) [65].

Figure 10.

Measured fecal coliform influent and effluent concentrations during the operational period.

4. Reuse potential and future perspective

By referring to the results of this study according to the tests carried out after the treatment process for the two constructed systems, and compared to the Palestinian standards for the re-use of treated wastewater, it is found that there is a compatibility between the results each of BOD, COD, TSS, Fecal Coliform that occurred in this study which is suitable for irrigation [64].

5. Conclusions

These types of CWs proved to be effective removal for pollutants and the effluent meets the Palestinian standards for wastewater reuse. The pollutant (COD, BOD, TKN, and Fecal Coliform) adsorption capacity of olivepomace charcoals is faster than citrus charcoal with olivepomace charcoal, but citrus charcoal with olivepomace charcoal gives more efficient removal with increasing HRT. Furthermore, removal capacity of olivepomace charcoal for TSS and P is faster and more efficient with increasing HRT than citrus charcoal with olivepomace charcoal. Using solid plant wood residues and solid olivepomace residues after burning is considered a good alternative substrate in CWs that are inexpensive and environmentally friendly, and gave better results compared to other organic substrates that have been used in CWs.

Acknowledgments

The author would like to express his gratitude to the first supporter Dr. Ziad Ibaid. The funder of this research is the Middle East Desalination Research Center (MEDRC); Facilitator staff for the completion of this study at Municipality of Gaza, and the Institute of Water and Environment- Al Azhar University of Gaza – Palestine. Special thanks go to colleagues, Dr. Khaldoun Abu Alhin Director of the Institute of Water and Environment, Dr. Mahmoud Shatat Assistant Professor in Water Desalination and Renewable Energy and Dr. Mazen Hamada Associate Prof. of Analytical Chemistry Chemistry Department at Al-Azhar University of Gaza–Palestine.