Abstract
The majority of cities in the Saharan territory of south Morocco utilize waste stabilization ponds (WSPs) for municipal wastewater treatment because of their relatively low capital, operational costs, and minimal complexity. New national effluent quality regulations have been implemented in Morocco, and they will be applied to all treatment systems. In this study, we chose three cities that are located in the Saharan area characterized by an arid climate and water scarcity. They are facing the challenges of rapid urbanization and the need to improve wastewater treatment and management. Treatment performance is impacted by community water use, pond design, and climate. The performance evaluation concerns seven physicochemical parameters during the year 2020. Monitoring results showed that WSPs in this climate can achieve removal rates between 75% and 85% for organic pollution and total suspended matter, but were challenged to produce effluent quality that meets reuse standards. Performance and statistical analysis have been done and confirm the existence of correlations between these parameters and the climate of the region. For the improvement of the quality of these waters, it is legitimate to upgrade the three WSPs with a tertiary treatment with maturation ponds.
HIGHLIGHTS
Waste stabilization ponds (WSPs) have relatively low capital, operational costs, and minimal complexity.
Arid climate and water scarcity in poor countries.
Effluent quality that meets reuse standards can be produced by WSPs.
Performance and statistical analysis have been done and confirm the existence of correlations between pollution parameters and the climate of the region.
Upgrading WSPs to meet reuse standards.
INTRODUCTION
The Saharan region of Morocco is marked by the scarcity of water and a harsh climate. The average annual precipitation is less than 50 mm, so this vast territory is entirely hyper-arid (Drake & Breeze 2016). The climate varies more according to the thermal regime than the rainfall. The closer we get to the Atlantic coast, the longer the climate remains ‘cool’. In this part of the country, the average maximum temperature is 42 °C and the minimum temperature is 18 °C throughout the year. However, this moderate climate is only true on a narrow strip of the coast because a little further inland, the desert influence is immediately felt (Born et al. 2008).
The development of the Saharan region is limited by the management of water resources and the implementation of alternative solutions to renovate sanitation. Waste stabilization ponds (WSPs) have become the main mode of treatment in these regions (Ho & Goethals 2020; Achag et al. 2021a; Ali et al. 2021; Edokpayi et al. 2021). The treated wastewater is mainly of domestic origin or from the agrifood sector; only one station serves the hospital environment. This implies that most of the effluents will not contain micropollutants which can limit their reuse in agriculture (Fatta et al. 2004; Salama et al. 2014; Battistelli et al. 2020; Jafer et al. 2020; Waheed et al. 2021). The stations were designed on the same principles: a pretreatment structure (75% of cases) followed by an anaerobic pond and a facultative microphyte pond. The McGarry or Marais models were used as the basis for sizing these stations. These models are based on the daily load.
The effectiveness of WSPs for the treatment of municipal wastewater has been demonstrated in temperate climates, and there are a number of guidance manuals for their design and operation (Adhikari & Fedler 2020; Jasim 2020; LeBlond et al. 2020; Letshwenyo et al. 2021). The use of WSP technology for wastewater treatment in desertic regions, however, is poorly understood, and it is unclear if design practices used in northern countries are applicable. The climate of Saharan regions likely poses the greatest constraint on treatment performance, but the influent wastewater quality and the operational regime will also influence the effluent quality (Ragush et al. 2017; Coggins et al. 2019).
There is limited understanding of how WSP systems function during the course of the year with respect to the level of oxygenation and treatment kinetics. The overall goal of this study was to assess the performance of three municipal WSPs in the Saharan region for the year 2020. The specific objectives of the research were to characterize the biogeochemical, environmental, and operational factors of WSP systems and determine the treatment performance and range of effluent quality depending on climate variations. This paper focuses on the removal of biochemical oxygen demand (BOD), chemical oxygen demand (COD), electrical conductivity (EC), total suspended solids (TSS), the fluctuations of pH, and water temperatures, as these are the parameters that will be regulated under the Moroccan municipal wastewater regulations.
METHODS
Study sites
Study communities’ size, location, and information
. | Elouatia . | Tantan . | Bouizakarne . |
---|---|---|---|
Location | 28°28′49.0″N 11°20′37.7″W | 28°26′36.1″N 11°06'02.8″W | 29°10′40.0″N 9°42'50.8″W |
Population | 9,295 | 60,698 | 14,228 |
Treatment level | Secondary AP + FP | Secondary AP + FP | Secondary AP + FP |
Flow (m3/day) | 290 | 2,652 | 759 |
Number of ponds | 2 AP 2 FP | 2 AP 2 FP | 4 AP 4 FP |
. | Elouatia . | Tantan . | Bouizakarne . |
---|---|---|---|
Location | 28°28′49.0″N 11°20′37.7″W | 28°26′36.1″N 11°06'02.8″W | 29°10′40.0″N 9°42'50.8″W |
Population | 9,295 | 60,698 | 14,228 |
Treatment level | Secondary AP + FP | Secondary AP + FP | Secondary AP + FP |
Flow (m3/day) | 290 | 2,652 | 759 |
Number of ponds | 2 AP 2 FP | 2 AP 2 FP | 4 AP 4 FP |
AP, anaerobic pond; FP, facultative pond.
Climate parameters of the region per month
Month . | Average temperature (°C) . | Average precipitation (mm) . |
---|---|---|
January | 26 | 7.7 |
February | 25 | 12.9 |
March | 27 | 12.4 |
April | 29 | 13.2 |
May | 30 | 8.4 |
June | 32 | 6.4 |
July | 34 | 5.8 |
August | 37 | 2 |
September | 35 | 2.2 |
October | 30 | 0.6 |
November | 28 | 1.4 |
December | 24 | 2.7 |
Month . | Average temperature (°C) . | Average precipitation (mm) . |
---|---|---|
January | 26 | 7.7 |
February | 25 | 12.9 |
March | 27 | 12.4 |
April | 29 | 13.2 |
May | 30 | 8.4 |
June | 32 | 6.4 |
July | 34 | 5.8 |
August | 37 | 2 |
September | 35 | 2.2 |
October | 30 | 0.6 |
November | 28 | 1.4 |
December | 24 | 2.7 |
Map of geographical locations of study communities (28.8360602, −10.7688009).
Satellite pictures of the three WSPs: (a) Bouizakarne, (b) Tantan, and (c) Elouatia.
Satellite pictures of the three WSPs: (a) Bouizakarne, (b) Tantan, and (c) Elouatia.
Wastewater discharged by the studied urban agglomerations passes through wastewater treatment plants with three stages of treatment: a pretreatment, an anaerobic treatment then facultative treatment, without a maturation pond, before being discharged into the natural environment, which consists mainly of rivers, above groundwater for Bouizakarne and Tantan, and into the ocean for Elouatia.
Sampling scenario
We proceeded to take samples at the level of the main wastewater collector which gathers all of the raw effluents at the entrance to the stations of wastewater treatment for each city, according to the following procedure:
– recording of data that influence the sample (climatic conditions, temperature, location, etc.);
– measure the water level and the hydraulic parameters before sampling;
– describe and identify all the samples taken;
– storage and transport;
– equipment decontamination (NF EN ISO 5667-1, NF EN ISO 5667-3, ISO 5667-10);
– collection of the sample in a container suitable for the analyses provided, description of the sample and measurement of in situ parameters (pH, temperature, conductivity, dissolved oxygen (DO) level, etc.); and
– appropriate storage of the sample.
The sampling points used to monitor the stations' performance were:
– entrance to the station;
– exit from the anaerobic ponds; and
– exit from the station.
We choose time-weighted composite samples which consist of point samples of equal volume taken at constant intervals during the sampling period.
Time-weighted composite samples are appropriate when the average quality of wastewater or effluents is of interest (for example, to determine compliance with a standard based on average quality or to determine average pollution of wastewater for the purpose of treatment process design).
Data collection timeline
The WSPs' wastewater quality was sampled in the year 2020, and the sampling sessions for each community are listed in Table 3. Because of the remoteness of the communities and the associated high travel costs, we decided to sample on the same day and split the cities between us. The four samples are representative of the four seasons as they occur in the middle of each season to capture longer-term, seasonal variations in climate. Each sampling is done manually over the course of 24 h and consists of a total of 12 samples, at the entry and exit of each pond.
Sampling type, position, and time for the three WSPs
Date of sampling . | Type of sampling . | Place . |
---|---|---|
15–16 January | Time-weighted composite: 12 samples each month | Entrance of the station, entrance of facultative ponds, and exit of the station |
15–16 April | ||
15–16 July | ||
15–16 October |
Date of sampling . | Type of sampling . | Place . |
---|---|---|
15–16 January | Time-weighted composite: 12 samples each month | Entrance of the station, entrance of facultative ponds, and exit of the station |
15–16 April | ||
15–16 July | ||
15–16 October |
Analysis methods
The present study aims to fill the data gap analysis in urban wastewater characteristics in Morocco and its statistical analysis. It is appropriate to conduct a statistical study on the physicochemical characterization of urban wastewater for better consideration in decision-making tools for the regulation of the sanitation sector. The physicochemical parameters studied were water and air temperature, pH, COD, biochemical oxygen demand for five days (BOD5), TSS, DO, and EC.
The pH and the temperature were determined by a CONSORT C831 type pH meter fitted with a temperature measuring probe (Moroccan Standard (ONEP 2019)). The BOD5 was determined by the OxiTop method (International Standard ISO 5815-2 (1/4/2003)). For COD and TSS, measurements were carried out respectively by the colorimetric method and the gravimetric method with a BAXTRANE type balance with a precision of 5 μg (Moroccan Standard (ONEP 2019)).
RESULTS AND DISCUSSION
Characterization of the environment
The Saharan climate is a significant factor in treatment efficacy, with average daily air temperatures in most cities exceeding 24 °C throughout the year. As a result, the WSPs' water temperatures averaged across depth were frequently exceeding 20 °C. These typical air temperatures would suggest a large potential for biological activity. The higher temperatures occurred during the day, when incident solar radiation was strong, causing the WSPs to warm up.
The wastewater temperature of the three WSPs
The average wastewater temperature of the three WSPs, Elouatia, Tantan, and Bouizakarne, during the year 2020.
The average wastewater temperature of the three WSPs, Elouatia, Tantan, and Bouizakarne, during the year 2020.
The raw wastewater from the city of Elouatia has a temperature range of 15–20 °C, with an average of 18 °C at the station's entry. There is a 2–4 °C rise in temperature from the station's entrance to the exit of the WSPs. These observed median temperatures were mostly attributable to the ocean's influence on the city's climate, and they fall within the range of limit values for direct discharge to the receiving environment and Moroccan requirements for irrigation water quality.
The temperature of raw wastewater from Tantan varies between 16.5 and 24 °C, with an average of 20 °C at the station's entry. An increase of 2–6 °C is measured from the entrance to the exit of the WSPs. The impact of the continental climate is mostly responsible for the high temperatures observed. It is a climate that affects areas distant from the coast or receiving winds from the interior of the continent which is in our case a desert. It is also distinguished by large yearly thermal amplitude.
The temperatures of raw wastewater from the city of Bouizakarne range between 18 and 23 °C, with an average of 21 °C at the station's entrance, and a decrease of 0.1–4 °C from the entrance to the exit of the WSPs. These unusual temperatures were primarily due to the effect of the cold wind blowing in from the nearby mountains.
The change in the difference between the three curves of the inlet, exit of anaerobic ponds, and outlet is explained in the first part by the effect of the pressure in the sewage network so the water entering the station is warmer than the water leaving, and they are also affected by the temperature of the air during the long retention time of the wastewater in the ponds (Achag et al. 2021a).
The temperature of the wastewater has a significant impact on the treatment process performance; for instance, at higher temperatures, settling is more effective. The biological activity that occurs during treatment also diminishes when the temperature drops (LaPara et al. 2001).
The wastewater pH of the three WSPs
The pH indicates the degree of acidity or basicity of a sample. It is calculated according to the concentration of H3O hydronium ions, and it depends on the origin and nature of the water. This parameter characterizes a large number of physicochemical equilibriums. The pH value alters the growth and reproduction of existing microorganisms in the wastewater. Most bacteria can grow in a pH range between 5 and 9, and the optimum is between 6.5 and 8.5; pH values below 5 or above 8.5 affect the growth and survival of aquatic microorganisms (Pearson et al. 1987).
The average wastewater pH of the three WSPs, Elouatia, Tantan, and Bouizakarne, during the year 2020.
The average wastewater pH of the three WSPs, Elouatia, Tantan, and Bouizakarne, during the year 2020.
According to practice, there is a high correlation between the alkalinity of treated wastewater and the density of microalgae, which can be explained by enhanced photosynthetic activity. Phytoplankton utilizing carbon dioxide has an influence on biological activity since the carbonate–bicarbonate balance is disrupted. During the summer treatment period, the WSPs in Elouatia and Tantan, where substantial phytoplankton productivity was recorded, had higher average pH of 8.3 and 9, respectively. These pH peaks were linked to WSPs with supersaturated oxygen levels and water samples that were clearly green, indicating a significant algal bloom. Bouizakarne, on the other hand, did not experience an algae bloom (Ali et al. 2020).
The wastewater EC of the three WSPs
Conductivity and salinity are measured in situ. The measurement of the conductivity of the water allows us to appreciate the quantity of salts dissolved in the wastewater (chlorides, sulfates, calcium, sodium, and magnesium) (de Sousa et al. 2014). The EC of wastewater depends primarily on the quality of the drinking water used, the diet of the population and industrial activities. Also it is more important when the temperature of the water increases.
The average wastewater EC of the three WSPs, Elouatia, Tantan, and Bouizakarne, during the year 2020.
The average wastewater EC of the three WSPs, Elouatia, Tantan, and Bouizakarne, during the year 2020.
In general, it is noted that the conductivity of the wastewater is high from the entrance of the station, and tends to increase at the level of the facultative ponds. Sometimes, a decrease is observed at the end of the WWTP (wastewater treatment plant), but it always remains higher than the limit value for discharge into the natural environment, which is 2,500 μS/cm.
The high conductivity values at the entrance of the station are due to the salinity of the drinking water upstream. At the pond's level, the degradation of organic matter by bacteria contributes to the production of nutritive salts such as nitrogen and phosphate; which results in an increase in EC at the station exit as is shown in Tantan WSPs (Adhikari & Fedler 2020).
The WSP system showed a varying trend of EC levels across the sampling months. Generally, higher EC levels were recorded in the effluent during the dry season, which could be attributed to evaporating effects, thus leading to higher levels of ions. In contrast, lower levels were determined in the effluent for January and February in the wet season. This could be due to dilution effects caused by the increased precipitation event. There is usually a breakdown of organic compounds and nutrients with the aid of microorganisms in WSP systems which could increase the levels of ionic compounds in the ponds. However, some WSP systems have reported a decrease in EC levels (Hodgson 2007; Levlin 2010; Gopolang & Letshwenyo 2018).
The wastewater BOD5 of the three WSPs
The average wastewater BOD5 of the three WSPs, Elouatia, Tantan, and Bouizakarne, during the year 2020.
The average wastewater BOD5 of the three WSPs, Elouatia, Tantan, and Bouizakarne, during the year 2020.
Given that BOD5 was measured on unfiltered samples, it is concluded that the high values observed were due to the significant proliferation of algae throughout the year. This is mainly due to abiotic factors such as increased temperature and sunshine (Gruchlik et al. 2018).
The high BOD5 removal between the raw wastewater and the treated wastewater in WSPs can be attributed to the settling processes, and the strong biological activity that occurs in ponds when water temperatures are above 20 °C. The concentrations of BOD5 at the exit of the WSPs are similar in all the three cities, and this confirms the effect of having the same pond design. WSPs promote the sequestration of organic matter at the bottom of ponds and reduce the organic matter that can be released into the water column after settling. The recent construction of the three WSPs means there would be minimal sludge accumulation. Both characteristics, design and young operational age, appear to limit the release of organic pollution into the receiving environment. Elouatia and Tantan WSPs also had lower BOD5 concentrations in the raw wastewater when compared with Bouizakarne, and the better raw water quality also may contribute to better WSP treated wastewater quality.
Raw wastewater quality in desert cities is generally stronger with respect to BOD5 concentrations when compared with wastewater quality entering wastewater treatment systems in the north (Abis & Mara 2003; Hind et al. 2013). Generally, this is attributed to the decreased per capita residential water use (in this desert region, local people consume 21 m3 of drinking water per person per year on average, while north Moroccans consume more than 50 m3 (Benassi 2008)). However, our results illustrate that even between these cities there can be significant differences in raw wastewater quality. There are challenges with respect to treating more concentrated raw wastewater as it can result in WSPs being overloaded with organic material, which appears to be the case (Hind et al. 2013).
The wastewater COD of the three WSPs
The average wastewater COD of the three WSPs, Elouatia, Tantan, and Bouizakarne, during the year 2020.
The average wastewater COD of the three WSPs, Elouatia, Tantan, and Bouizakarne, during the year 2020.
Similar results have been reported in several WSP systems in developing countries (Mara 2004; Olukanni & Ducoste 2011). Although there have been reported cases of COD reductions up to 70%, the resulting effluents usually exceed the regulatory standards of most countries (50–75 mg/L). In this study, the main reason for this finding could be the fact that the three WSP facilities have been overstretched beyond their design capacity and receive more than twice the wastewater they were designed to handle and treat. Another contributing factor could be the reduction of hydraulic retention time (HRT) (Ho et al. 2017).
The wastewater TSS of the three WSPs
Solids removal is relatively successful in desertic WSPs systems, with most systems generating effluent quality of about 80 mg/L TSS. Some WSP systems have been found to have difficulty fulfilling effluent TSS standards due to phytoplankton bloom. This is crucial to examine since system designs that enhance phytoplankton development will almost certainly result in an effluent with high TSS levels if extra phytoplankton removal procedures are not used.
The decline in the yield of the STEP (wastewater treatment plant) of Bouizakarne can be explained by the ageing of the basins and the collection of sludge at the bottom of the basins, which necessitates a cleaning operation. For the Elouatia WWTPs, we notice that the yield improves over time. This improvement is due to the phenomenon of groundwater infiltration, which causes the purified water to be diluted by the water infiltrated into the basins, resulting in an increase in yield and a decrease in concentrations released.
The wastewater DO of the three WSPs
DO is an essential component of water because it supports the life of wildlife and it affects the biological reactions that take place in aquatic ecosystems. The solubility of oxygen in water depends on various factors, including the temperature, pressure, and ionic strength of the water (Kayombo et al. 2000).
Ratios and characterization parameters of wastewater
Wastewater can be classified into two categories: biodegradable and non-biodegradable. The biodegradability of the effluent is defined by calculating the coefficient of biodegradability of raw wastewater effluents. This coefficient is calculated by the COD/BOD5 ratio and depends on the origin and nature of the wastewater, which can be domestic or industrial, which requires different treatments (Arnell et al. 2017; Achag et al. 2021a).
The COD/BOD5 ratio is very important for assessing the biodegradability of wastewater in the treatment plant. During the study period, the values recorded for the three WSPs are below 3 and the average value of the COD/BOD5 ratio is 1.34 ± 0.14, which means that the effluents of the three cities are easily biodegradable and the dominant character is that of domestic wastewater. The values obtained confirm the absence of industrial discharge connected to the sewerage network. If it is between 3 and 7, the wastewater can hardly be biodegradable. It was also noted that the biodegradability coefficient does not rise from the inlet to the outlet. In fact, this coefficient does not change much between the inlet and outlet. This could mean that the effluent is not yet stabilized and more organic matter could be biodegraded, also other factors could weigh in such as the presence of algae or the elimination of non-biodegradable matter by some mechanism other than bacterial degradation, such as adsorption.
We also considered the BOD5/COD ratio, which gives very interesting insights into the origin of the wastewater pollution and its treatment options. The values recorded during the study period ranged from 0.69 to 0.90; these values show a very high organic load and confirm the possibility of an easily biological treatment, being higher than 0.3. The BOD5/COD ratio indicates a dominance of organic matter.
The average TSS/BOD5 ratio is 0.51–0.8, which is relatively low compared with the usual value (between 1.2 and 1.5). This ratio provides additional information about organic matter quantities in the effluent, but also provides information about sludge production. The low concentrations are due to the rapid sedimentation of the suspended matter (Abdouni et al. 2021).
TREATMENT PERFORMANCE
To the best of our knowledge, our study is the first to perform an extensive one-year study of multiple WSP stations located in the Saharan desert. Other studies have examined the performance of WSPs in arid regions (Mandi et al. 1993, 1998; Chaoua et al. 2018), but the systems have been functionally different and the climate significantly warmer in our case.
Elouatia, Tantan, and Bouizakarne are three plants with WSP treatment process and they are situated in the same region, a desert south of Morocco, but they differ by their location according to the ocean. They have approximately the same anthropic and industrial activities.
Table 4 presents the average abatement rate of each station during one year of monitoring. These results allow us to compare and to understand all the anomalies, especially the increase of the EC and the TSS from the facultative ponds to the exit of the plant. This phenomenon is similar in two out of the three plants. The performance of WSPs depends on water and air temperatures, the depth of the ponds and the nature of the pollution entering (Ali et al. 2021), and these factors are almost identical in the three stations.
Abatement rates of Elouatia, Tantan, and Bouizakarne wastewater treatment plants
Ponds . | Anaerobic . | Facultative . | Total . | |
---|---|---|---|---|
Tw | Elouatia | +2.92% | +2.85% | +5.88% |
Tantan | +2.92% | +5.71% | +8.82% | |
Bouizakarne | −8.57% | −12.5% | −20% | |
pH | Elouatia | +0.23% | −0.95% | −0.71% |
Tantan | +7.96% | +6.28% | +14.75% | |
Bouizakarne | −1.51% | +8.83% | −7.61% | |
EC | Elouatia | −16.38% | +4.64% | −12.5% |
Tantan | +2.53% | +27.59% | +24.36% | |
Bouizakarne | −5.75% | −4.19% | −9.71% | |
BOD5 | Elouatia | −83.33% | −10% | −85% |
Tantan | −73.07% | −10.71% | −75.96% | |
Bouizakarne | −43.24% | −48.8% | −70.94% | |
COD | Elouatia | −47% | −44% | −77% |
Tantan | −42% | −55.53% | −74.23% | |
Bouizakarne | −57% | −50% | −70% | |
TSS | Elouatia | −92% | +20% | −78.04% |
Tantan | −55% | −56% | −80.64% | |
Bouizakarne | −79% | +18.42% | −75.27% |
Ponds . | Anaerobic . | Facultative . | Total . | |
---|---|---|---|---|
Tw | Elouatia | +2.92% | +2.85% | +5.88% |
Tantan | +2.92% | +5.71% | +8.82% | |
Bouizakarne | −8.57% | −12.5% | −20% | |
pH | Elouatia | +0.23% | −0.95% | −0.71% |
Tantan | +7.96% | +6.28% | +14.75% | |
Bouizakarne | −1.51% | +8.83% | −7.61% | |
EC | Elouatia | −16.38% | +4.64% | −12.5% |
Tantan | +2.53% | +27.59% | +24.36% | |
Bouizakarne | −5.75% | −4.19% | −9.71% | |
BOD5 | Elouatia | −83.33% | −10% | −85% |
Tantan | −73.07% | −10.71% | −75.96% | |
Bouizakarne | −43.24% | −48.8% | −70.94% | |
COD | Elouatia | −47% | −44% | −77% |
Tantan | −42% | −55.53% | −74.23% | |
Bouizakarne | −57% | −50% | −70% | |
TSS | Elouatia | −92% | +20% | −78.04% |
Tantan | −55% | −56% | −80.64% | |
Bouizakarne | −79% | +18.42% | −75.27% |
BOD removal efficiency of the facultative ponds in the three WSPs is poor, because of over-surface BOD5 loading; consequently, the oxygen produced is not sufficient for the degradation of all organic materials. Effluents which have high concentration of BOD and COD can cause depletion of oxygen in the aquatic environment or in the receiving water bodies. Therefore, the BOD and COD removal and the consequent quality of the effluent depend on the amount of oxygen present, retention time and temperature of the ponds (Gruchlik et al. 2018). However, seasonal variations in organic micropollutant removal are mostly evident in this arid climate compared with temperate (Matamoros et al. 2016).
Winter water temperatures were observed to be about 12–13 °C lower than the summer temperatures. Although the temperature of the anaerobic ponds at different depths is not available, it is possible that the temperature of the anaerobic pond at the lower depths might be higher than the pond surface temperature or that of the raw influent. This is because the anaerobic process itself is an exothermic process and the heat retention must be further aided by long sludge retention time, thereby likely favoring BOD and COD removal even though the surface temperature of the pond water is low during the cold winter period.
Temperature plays an important role in WSPs with respect to the activity of algae and bacteria and can impact the removal of micropollutants in WSPs, especially via biodegradation, with increased removals typically reported during warmer months (Li et al. 2013; Matamoros et al. 2015). For example, Li et al. (2013) observed higher removal efficiencies in a US lagoon system in September (average temperatures of 20 °C) than in November (average temperatures of 4 °C), while Matamoros et al. (2015) observed that the removal of both biodegradable compounds and moderately volatile compounds were better in the summer (average temperature of 26 °C) compared with winter (average temperature of 11 °C). Biodegradation is usually optimized at the physiological temperature of the microorganism, as this promotes optimal activity. For example, the optimum temperature for biodegradation rates for the antibiotic ceftiofur was found to be between 35 and 40 °C, close to the optimum temperature for gut microorganisms of 37 °C (Qu et al. 2019). Light plays a major role in the performance of WSPs by providing the energy source for photosynthesis (Davies-Colley et al. 2005; Heaven et al. 2005) and also provides a major pathway for micropollutant degradation through direct or indirect UV (200–400 nm) photodegradation (Rivera-Utrilla et al. 2013). The rate of photodegradation of a particular compound is influenced by the variation in the intensity of solar irradiance with both latitude and season (Andreozzi et al. 2003)). For example, Matamoros et al. (2015) found that the removal of photodegradable compounds was better in summer when the average daily solar irradiation was 282 W m2, compared with winter when the solar irradiation was much lower (74 W m2).
Algae usually contribute about 70%–90% of the BOD, COD, and TSS, and from the fact that the above effluent results are obtained with unfiltered samples, the performance of the three plants in organic removal is still higher. Although the data on the algae are not available for these ponds, it is highly likely that the wastewater effluents from the facultative ponds contain algae (Pham et al. 2014). This is because the presence of nutrients in the facultative pond water aided by sunlight and a slight atmospheric mixing at the pond surface could provide conditions for the growth of a certain amount of algae. The visual appearance of algal growth in the facultative pond especially during the warm summer months was confirmed.
DISCUSSION
The interpretation of the data collected at the three installations demonstrates that the technique of stabilization ponds is a solution compatible with certain discharge quality objectives, particularly due to a high reduction of suspended solids and carbon pollution during high-temperature periods (between 75% and 85%). An evaluation of the results reveals that waste stabilization ponds in this region impose a short residence time of the effluents contrary to other waste stabilization ponds in Europe and the Mediterranean (Camors, Hoedic, Houat, Locoal-Mendon), the three stations of Locmaria (Le Skeul, Borderhouat, Le Grand Cosquet), and Plumergat and Pluvigner-Bieuzy (Racault & Boutin 2005; Abdel-Shafy & Salem 2007; González-Zeas et al. 2014), and are subject to a low seasonal effect. Clear patterns may be recognized, which should allow for the optimal field of use for stabilization ponds in Saharan regions.
The combination of concentrated wastewater and an increase in temperature in WSPs may result in sludge resuspension in the first pond due to a fermentation phenomenon. The rise in temperature, along with the lack of wind, promotes thermal stratification; heating will occur mostly in the upper layer of well-oxygenated water, masking fermentation aromas. A protracted heatwave promotes evaporation, which can cause the water level to drop and the flow between ponds to stop.
TSS in wastewater can absorb heavy metals during treatment and introduce heavy metals into the receiving environment (Nkegbe et al. 2005). In the three WSPs, suspended solids did not have enough time to settle and hence escaped into the receiving environment. An increase in TSS could have been a result of algae dying that were present in the ponds; this was reported by Dias et al. (2014), who attributed the increase of TSS in pond effluent to the presence of algae. The same observation was reported by Abdel-Shafy & Salem (2007), who concluded in their study in Egypt that the algae present in ponds were responsible for increasing TSS concentration in the effluent. Also, pond desludging as well as re-positioning of inlet–outlet increased the removal efficiency of both BOD and TSS. The significant removal of TSS can be achieved through desludging as it was observed that the Bouizakarne WSP had high sludge accumulation and hence a short-circuiting of wastewater resulting in poor TSS settling.
Given the residence time of the water and the inertia of its response, WSPs are particularly well adapted to the treatment of hot-climate wastewater. A sudden hydraulic overload in the summer, on the other hand, might result in a large flow of algae, which can have a negative influence on sensitive environments. This washing effect can be reduced by installing a flow limiter at the pond exit (for example, a floating weir). In order to boost disinfection rates and remove more TSS and COD, maturation ponds must be added (Tanner et al. 2005; Bracho et al. 2006).
PRINCIPAL COMPONENT ANALYSIS (PCA)
To better interpret the results obtained relating to physicochemical parameters, a statistical study was made using a multivariate analysis method: principal component analysis (PCA), using the XLSTAT software. The objective of the PCA is to present, in a graphical form, the maximum of the information contained in a data table, based on the principle of double projection on the factorial axes. Data processing is by PCA, using Tair, Tw, pH, Cond, DO, BOD5, COD, and TSS and as individuals during the months of sampling.
Analysis of the results shows that most of the information is explained by the first two factorial axes. In the factorial plan F1 × F2, the two components F1 and F2 contribute to the total inertia with a percentage of 98.23% of the total data set (Table 5).
Factorial data processed and represented percentages
. | F1 . | F2 . | F3 . |
---|---|---|---|
Own value | 7.596 | 0.262 | 0.142 |
Variability (%) | 94.954 | 3.275 | 1.771 |
Cumulative% | 94.954 | 98.229 | 100.000 |
. | F1 . | F2 . | F3 . |
---|---|---|---|
Own value | 7.596 | 0.262 | 0.142 |
Variability (%) | 94.954 | 3.275 | 1.771 |
Cumulative% | 94.954 | 98.229 | 100.000 |
The correlation matrix (Pearson (n))
Variables . | Tair . | Tw . | pH . | EC . | BOD5 . | COD . | TSS . | DO . |
---|---|---|---|---|---|---|---|---|
Tair | 1 | 0.911 | 0.989 | −0.915 | −0.994 | −0.991 | −0.938 | −0.996 |
Tw | 0.911 | 1 | 0.872 | −0.987 | −0.863 | −0.942 | −0.925 | −0.944 |
pH | 0.989 | 0.872 | 1 | −0.899 | −0.984 | −0.985 | −0.878 | −0.980 |
EC | −0.915 | −0.987 | −0.899 | 1 | 0.863 | 0.957 | 0.874 | 0.946 |
BOD5 | −0.994 | −0.863 | −0.984 | 0.863 | 1 | 0.970 | 0.929 | 0.980 |
COD | −0.991 | −0.942 | −0.985 | 0.957 | 0.970 | 1 | 0.918 | 0.996 |
TSS | −0.938 | −0.925 | −0.878 | 0.874 | 0.929 | 0.918 | 1 | 0.947 |
DO | −0.996 | −0.944 | −0.980 | 0.946 | 0.980 | 0.996 | 0.947 | 1 |
Variables . | Tair . | Tw . | pH . | EC . | BOD5 . | COD . | TSS . | DO . |
---|---|---|---|---|---|---|---|---|
Tair | 1 | 0.911 | 0.989 | −0.915 | −0.994 | −0.991 | −0.938 | −0.996 |
Tw | 0.911 | 1 | 0.872 | −0.987 | −0.863 | −0.942 | −0.925 | −0.944 |
pH | 0.989 | 0.872 | 1 | −0.899 | −0.984 | −0.985 | −0.878 | −0.980 |
EC | −0.915 | −0.987 | −0.899 | 1 | 0.863 | 0.957 | 0.874 | 0.946 |
BOD5 | −0.994 | −0.863 | −0.984 | 0.863 | 1 | 0.970 | 0.929 | 0.980 |
COD | −0.991 | −0.942 | −0.985 | 0.957 | 0.970 | 1 | 0.918 | 0.996 |
TSS | −0.938 | −0.925 | −0.878 | 0.874 | 0.929 | 0.918 | 1 | 0.947 |
DO | −0.996 | −0.944 | −0.980 | 0.946 | 0.980 | 0.996 | 0.947 | 1 |
Biplots of the two axes of PCA performed on physicochemical variables.
The air temperature is correlated:
positively with the water temperature and pH (r = 0.911; r = 0.989);
negatively with EC, BOD5, COD, TSS, and DO (r = −0.915; r = −0.994; r = −0.991; r = −0.938; r = −0.996), indicating that the removal of organic and suspended matter in wastewater is strongly linked to the rise of temperature and pH.
Furthermore, BOD5, COD, TSS, EC, and DO are all positively correlated, indicating that these parameters increase or decrease at the same time.
The projection of the analyses values of the physicochemical parameters on the factorial plane F1 × F2 corroborates the existence of two phenoma:
The influence of temperature of air and water on the treatment performance, as at higher temperatures more BOD5, COD, and TSS are eliminated and the degradation of the organic pollution is increased significantly, but the DO is decreased and this is due to the consumption of bacteria responsible for the degradation of organic matter and also the low solubility of oxygen at high temperatures.
The relationship between pH and high temperatures, since higher temperatures result in increased algae development, especially in facultative ponds where pH rises.
The interpretation of the data collected on the installations spread over the whole year of 2020 shows that the temperature of the wastewater greatly influences the efficiency of the treatment process. Settling is more efficient at higher temperatures in summer, and the biological activity that takes place during treatment increases in hot climates, but oxygen dissolves much more in low temperatures. Temperature also influences the speed of chemical and biological reactions. It affects the level of DO in water (Chaturvedi et al. 2014), photosynthesis in aquatic plants, metabolic rates of aquatic organisms, and the susceptibility of these organisms to pollution, pests, and diseases.
IMPLICATIONS AND RECOMMENDATIONS FOR MANAGERS OF WSP SYSTEMS IN DEVELOPING COUNTRIES
WSP wastewater is commonly utilized in irrigation. However, uncontrolled reuse causes health risks and soil deterioration. The presence of WSP in Morocco has frequently supported wastewater reuse in unrestricted irrigation. Poor populations near wastewater treatment plants rely on wastewater irrigation for a living. The reuse of the WSP effluent in irrigation of crops, especially vegetables, has often raised public outcry (Agunwamba 2001). To forestall the potential risk that inadequate wastewater treatment would have on the environment and public health, it is imperative that WSP systems functions efficiently. The three WSP systems of our study, like many others in developing countries, do not efficiently treat the wastewater they receive, for several reasons.
We recommend the authorities of the three cities to have a plan aimed at extending and upgrading the recent WSPs stations that only have anaerobic and facultative ponds. Upgrading the performance of the existing WSPs to achieve the appropriate treatment required for reuse systems and discharge to the environment is very necessary, and in addition, the use of maturation ponds as being the best cost-effective systems for wastewater treatment prior to reuse (Jagals & Lues 1996; Von Sperling 2005; Achag et al. 2021b).
Maturation ponds (low-cost polishing ponds), which generally follow the secondary facultative pond, are primarily designed for tertiary treatment, i.e., the removal of pathogens, nutrients and possibly algae. They are very shallow (usually 0.9–1 m depth), to allow light penetration to the bottom and aerobic conditions throughout the whole depth. In the absence of effluent limits for pathogens, maturation ponds act as a buffer for facultative pond failure and are useful for nutrient removal. They will also produce an effluent suitable only for restricted irrigation. Therefore, additional maturation ponds will only be needed if a higher-quality effluent is required for unrestricted irrigation (Dias et al. 2014).
EFFECTS OF CLIMATE CHANGE ON THE TREATMENT EFFICIENCY
A rise in the number of people living in cities puts additional strain on water supplies and the environment. On the one hand, this entails greater water consumption and increasing sewage generation, which demands treatment to prevent health hazards and environmental harm. On the other hand, treated municipal wastewater provides a reliable and high-quality source of water. The benefits of that extra, consistent source of water, however, are hampered by wastewater collection, treatment, and disposal, all of which are capital- and energy-intensive operations. This creates a costly societal challenge that must be solved by good public policy.
For decades to come, climate change is being proclaimed as unavoidable (increased temperatures, and more frequent and more pronounced extreme weather phenomena such as heavy rain, storms, and drought) (Tram VO et al. 2014; Valipour 2017). This research aids in determining the impact on sanitation systems: as in Saharan regions, little attention has been paid to the increase in the ambient temperature as it was commonly admitted that a warmer climate would accelerate the reaction kinetics, thus reduce the energy requirements. However, the warmer temperature was reported to create favorable conditions for corrosion of raw wastewater pipelines with the formation of hydrogen sulfide. In addition, it increased the fermentation of solids in the sludge thickeners, which caused odor issues.
WSPs are normally considered more robust with the fluctuation of the influent, however, scenario analysis is needed for a more comprehensive assessment. The temperature increase can be another opportunity for WSP application. As a natural-based system, the biological treatment processes inside the ponds are considerably influenced by temperature. It was recorded that the surface loading rate of ponds increased almost 2.5 times when changing from temperate regions to the tropical and subtropical climate (Ho et al. 2019). Higher temperature means that there is a lower land requirement and higher removal and energy efficiency, which can possibly put pond treatment systems in an advantageous position in competition with other treatment technologies.
The implications on wastewater infrastructure can be classified as follows:
indirect effects of climate change, such as reduced water use due to water conservation;
the direct consequences of climate change on infrastructure.
Climate change effects on WWT processes may be limited by employing an impact assessment tool, monitoring WWTPs, and using vulnerability as an effective way of identifying a utility's priority concerns in respect to climate change.
On the other hand, the WWT contributes to climate change by producing GHGs such as carbon dioxide (CO2) from aerobic (oxidation) processes, methane (CH4) from anaerobic processes, and nitrous oxide (N2O) as an intermediate product from NDN processes that can be emitted to the atmosphere.
Future adaptation techniques and expertise to regulate wastewater cycle emissions appear to be required, and the vulnerability climate assessment should be improved to interact with adaptive actions that might address emission sources. Furthermore, recovering energy to produce heat and power for the WWTP process by employing anaerobic sludge digestion biogas can offset large fossil-fuel-related GHG emissions (CH4 and so on).
CONCLUSION
The physicochemical properties of three different urban wastewaters in Sahara were evaluated in order to serve as a decision-making tool for integrated effluent management in the Moroccan Sahara. The findings revealed that the physicochemical characteristics of wastewater are mostly determined by its kind, source, and geographical location. Data analysis revealed a substantial variation according to effluent typology and a reasonable association between the organic contamination indicators of each effluent.
This study looked at the treatment performance of WSPs used in the Sahara. WSP systems function as controlled discharge storage ponds in this harsh climate, with treatment methods confined to relatively short time-periods (approximately 25 days) and ambient temperatures ranging from 20 to 45 °C. In this location, extended photoperiods warm the surface water layers, resulting in surface water temperatures ranging from 16 to 24 °C. The examined WSPs were tested in order to meet tertiary wastewater treatment targets for BOD5 and TSS elimination.
Current desert WSP design standards should be revised to verify that WSPs operate in the desired oxygen concentrations and achieve treatment standards. Since stricter regulations of wastewater discharge and reuse are being enforced and the current treatment plant does not achieve the Moroccan norms for irrigation, this low-cost technology is now facing a crossroads of either being upgraded or being substituted. Considering the current and future challenges of water shortage and pollution, it is difficult to overstress the importance of existing assets of pond treatment technology, especially in the developing world where economic affordability is still a bottleneck for wastewater treatment installation.
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.
CONFLICT OF INTEREST
The authors declare there is no conflict.