Study the Effectiveness of the Natural Lagoons for Removal of Organic Matter, Nutrients and Fecal Coliform from Urban Domestic Wastewater under Arid Climate (Morocco)

Morocco is one of the countries most affected by the scarcity of water resources and the poor distribution of rain - fall. Natural lagoon is the most widely used treatment process in Morocco. Indeed, Morocco is a sunny country throughout the year. The lagoon system requires minimal effort in operation and maintenance. Moreover, this sys - tem is the best process for removing bacteria indicators. For these reasons, a study of the purification performance of natural lagoons during five years, located in Chichaoua region (Morocco) was carried out. The lagoon system was monitored for five years, with measurements taken every three months at the input and output of the system. It received a hydraulic loading rate (HLR) between 1171 and 2760 m 3 with an average of 2053 m 3 /day. The ob - tained results show the removal of 37% of TSS, 63% of BOD 5 , 60% of COD, 61% of NH 4+ , 37% of TP, and 6.5 log units of coliforms during the mentioned monitoring period. Thus, the effluent values do not always comply with the Moroccan water quality requirement for irrigation reuse. Additionally, the statistical analyses confirm the pres - ence of a strong linear correlation between almost all variables in the lagoon output. Therefore, the performance efficiency of the lagoon in removing organic matter and nutrients was significantly (p < 0.05) dependent on the season. However, the fecal contamination was not significantly (p > 0.05) affected by the seasons in this study. At the end of this study a complementary treatment before the reuse of treated wastewater, was proposed.


INTRODUCTION
The water resources in Morocco are affected qualitatively through industrial activities [Benaddi et al., 2022a; Benaddi et al., 2022b] and quantitatively by human activities, agricultural and industrial activities focused on satisfying food demands have also increased [Chaoua et al. 2018; Mancosu et al. 2015]. The use of water resources is at an all-time high in arid and semi-arid countries, including Morocco, due to recent irregular rainfall patterns, frequent droughts, and other environmental factors, consequently, water is becoming a precious resource. The reuse of wastewater is a menace to public health, the environment, and ecological stability. Therefore, wastewater treatment is an urgent necessity to protect water resources and reduce the water shortage problem, and it is a viable option for agriculture in developing countries. Moroccan urban areas Study the Effectiveness of the Natural Lagoons for Removal of Organic Matter, Nutrients and Fecal Coliform from Urban Domestic Wastewater under Arid Climate (Morocco) discharged 550 Mm 3 per year of wastewater volume, and only 45% was treated by 117 plants. Therefore, the goal is to reuse 325 Mm 3 of treated wastewater in irrigation in 2030 (Morocco ministry of agriculture). The natural lagoon is a viable solution for rural and urban communities, since it is an eco-friendly wastewater treatment technique [Laidia Zerkaoui et al., 2020]. It is good landscape integration and a low-cost treatment method that produces effluents of good quality and free of various pollutants. Furthermore, the lagoon is easy to operate and maintain compared to other treatment methods. Several studies have shown that the lagoon system effectively decreased the majority of wastewater contamination, including suspended particles, organic matter, and nutrients [Allaoui et al., 2016;Laidia Zerkaoui et al., 2020]. Additionally, the presence of fecal contamination in domestic wastewater, including fecal coliform, might cause serious environmental problems [Nacir et al., 2012]. The objective of this work was to examine the performance of a lagoon treatment plant located in the Chichaoua region (Morocco) for treating domestic wastewater for five years taking into account the Moroccan irrigation standards for reuse and to evaluate the seasonality effects on the performance of lagoon system under arid climate, then propose an appropriate complementary treatment.

Study site
Sampling was done at the Chichaoua wastewater treatment plant located in central Morocco (313238N; 84558W) ( Fig. 1). The WWTP has a capacity of 22,000 equivalent inhabitants. It operated under 2053 m 3 /day. Natural lagoons are the raw water treatment method used in this study. The first step is screening, and then the ponds are arranged and operated in series, with anaerobic ponds preceding the facultative ponds, which then feed into several maturation ponds.

Sample collection and analyses
Water sampling was conducted at the input and output of natural lagoons during five years from 01/2017 to 01/2021 with one sample per three months. They are stored in sterile glass bottles for microbial and physicochemical analysis.
The bottles were transported in a cooler at a temperature of 4 °C to the laboratory analysis. The physicochemical parameters measured in the laboratory were Temperature, pH, Turbidity, Electrical conductivity, and dissolved oxygen, which were measured using a HI 9829 multi-parameter probe (HANNA, Woonsocket, RI, USA). Total suspended solids (TSS) were determined after filtering a sample through a Millipore filter (0.45 µm) and drying the retained residue at 105°C for 120 min (AFNOR-T90-105) [AF-NOR, 1997]; COD was determined by digestion followed by colorimetric dichromate method (AF-NOR-T90-101) (AFNOR, 1997); The biological oxygen demand (BOD 5 ) is determined according to the respirometry method T90-103 of AFNOR (1983). Total phosphorus (TP) was performed by molybdate and ascorbic acid method after potassium peroxydisulfate digestion (AFNOR-T90-023) (AFNOR, 1997). NH 4+ was determined by the indophenol method (AFNOR-T90-015) [AFNOR, 1997]; fecal coliforms (FC) count was performed according to the AFNOR Standard NF EN ISO 9308-1 [AFNOR, 1997] in TTC Tegretol medium. The dishes were incubated at 44.5 °C for FC for a period of 24 h and then the number of forming colony units was calculated.

Statistical analyses
To visualize the effect of the season on the lagoon-treated water, the data were subjected to principal component analysis (PCA). Then, Pearson linear correlation was applied to show the correlations between several parameters at the output of the lagoon.

Coefficient of biodegradability (COD/BOD 5 )
The COD/BOD 5 ratio for raw wastewater varies between 1.67 to 3.42 with an average of 2.15. According to [Ouafae El Hachemi and Hassan Elhalouani Salgot, 2012], biodegradability coefficients are calculated by the COD/BOD 5 ratio and depend on the nature and origin of the wastewater which can be domestic or industrial, the COD/BOD 5 ratio for raw domestic wastewater is generally between 1.25 and 2.5 and can be more than 10 for industrial wastewater, which requires different treatments. It can be observed that the coeffi cient of biodegradability has been evaluated at 3.42 in this study. This result confi rms the presence of industrial discharge connected to the domestic wastewater network.

In situ parameters
The performance of natural lagoons was monitored for fi ve years from January 2017 to January 2021. Figure 2 showed the variation of pH, DO, Temperature and EC at the input and output of the studied system. The obtained results for dissolved oxygen ( Fig. 2A) show that the extreme minimum and maximum values observed at the inlet of the station were between 0.1 mg/l and 0.22 mg/l with an average concentration of 0.16 mg/l, while those at the outlet range between 0 mg/l and 3.92 mg/l with an average concentration of 1.16 mg/l. The presence of dissolved oxygen positively affects the aerobic degradation reactions of organic matter and more generally the biological balance of the hydraulic media. The oxygen measured in There is a slight increase in pH in the treated water, which can be explained by the nitrification reaction that releases H + ions [Ech-Chafay et al. 2020]. Additionally, the pH value was between (6.5 -8.4) which is the admissible limit for wastewater reuse according to Moroccan Standards for reuse. The value of the wastewater temperature varied between 18.2 °C to 29.6 °C with an average of 21.53 °C, and for wastewater treated at the lagoon outlet was varied between 17 and 27.3 with an average of 21.08 (Fig. 2c). The difference between the input and output temperatures values was not statistically significant (p < 0.05). The conductivity of water directly indicates its salinity (United Nations Food and Agriculture Organization (FAO) 2003). It can be seen (Fig.2D) that the conductivity of the wastewater is high at the inlet of the lagoon (2597 to 4027 μs/cm), and a decrease is at the outlet of the lagoon (2126 to 3757 μS/cm), it is still lower than the admissible limit value for wastewater reuse, which is 12000 μS/cm (Moroccan Standards 2006) but higher than the admissible value recorded by FAO which is 700 μS/cm. The high conductivity values at the inlet of the WWTP are due to the salinity of the upstream drinking water. At the WWTP, the degradation of organic matter by bacteria contributes to the production of nutrient salts, such as nitrogen and phosphate; these results are reflected in the increase of conductivity. As a result of the assimilation of these salts by the algae, there could be a reduction of conductivity at the outlet of the lagoon.

Removal of organic pollution
The concentration of TSS in the wastewater varied between 246 and 484 with an average of 351.79 mg/l. Furthermore, TSS output ranged between 124 and 387 mg/l with TSS removal efficiency of 60% with an average of 37%. The reduction of TSS concentration in the lagoon is due to the sedimentation of the wastewater in the lagoons [Boutayeb and Bouzidi 2013] but the value of TSS in the treated wastewater is above the required standard of FAO (20 mg/l) and Morocco admissible for wastewater reuse (100 mg/l). This may be due to the presence of algae in the treated water [Chachoua et al., 2013]. The BOD 5 value is an indication of the amount of oxygen that is used for the destruction of decomposable organic matter by biochemical processes [Türker Okaygün,and Almaqadma, 2009]. The BOD 5 concentrations at the inlet range from 360 mg O 2 /l to 780 mg/l with an average of 535.29 mg O 2 /l ( Figure 3B), those at the outlet range from 15.8 mg O 2 /l to 400 mg O 2 /l with an average of 184.82 mg O 2 /l. The average COD concentration varies between 310 and 1075 mg/l at the output and the removal efficiency reaches 69% (Fig. 3C). The BOD 5 yields are higher than the COD yields, since the natural lagoon treatment system considerably reduces the biodegradable part of the effluent [Boutayeb and Bouzidi, 2013].

Nitrogen and phosphorus removal
According to Figure 4, the concentration of ammonium at the input of the lagoon is varied between 105.12 and 348.89 mg/l with an average of 192.51 mg/l, for the output, it ranged between 19.73 and 108 mg/l with 72.15 mg/l. This result indicates a degradation of ammonium concentrations up to 61%. The presence of algae in the basins provides a support for the growth of bacterial communities, thus providing the formation of a biofilm that contributes to the degradation of pollutants. Additionally, development of nitrifying bacteria, which are responsible for the nitrification of ammonium under aeration condition [Boutayeb and Bouzidi, 2013]. The ammonium removal was decreased from 86% in 2017 to 43% in 2021; this result can be due to the lower nitrification process over time. However, the average concentration of nitrate was 2.4 mg/l during 2017 and 0.82 mg/l during 2021. [M. Driche, 2010] reported that the nitrates formed by the nitrifi cation process diff used into the lower layers, where they transformed into nitrogen gazes (N 2 ) later by denitrifi cation processes. The removal effi ciency of TP ranged between 15 and 59% with an average of 37%. Usually, polyphosphates undergo hydrolysis and are converted to orthophosphate form; this process is usually very slow .

Removal of fecal contamination
The results of the bacteriological analyses at the input of the treatment plant revealed the presence of fecal contamination. According to Figure 5, fecal coliforms varied between 2.2·10 6 and 5.9·10 6 CFU/100 ml with an average of 3.6·10 6 CFU/100 ml at the input. In turn, fecal coliforms varied between 10 5 and 4.5·10 5 CFU/100 ml with an average of 3.4·10 5 CFU/100 ml at the output of the lagoon system. The reduction of indicator bacteria can be due to various physico-chemical and/or biological processes, namely infi ltration by sedimentation; competition with native species, predation by bacteriophage species (zooplankton), production of natural inhibiting or bactericidal substances (e.g. antibiotics) by certain bacteria and micro-algae, leading to the death of or a reduction in the reproduction of indicator germs [Ndip et al., 2008;Zidan et al., 2022]. However, the lagoon treatment adopted exceeds the values destined for irrigation (1000 CFU/100 ml).

Eff ect of season period on the treated water
Principal component analysis (PCA) was realized to study the impact of seasons on the water quality at the output of the lagoon by grouping individuals based on the study parameters dataset (Fig.7). The fi rst dimensions represent 39.3% of the total variability dataset combined with the . The TSS contributes positively to Dim 2 (cos > 0.75), but there is a significant negative relationship with the four individuals during the summer and a negligible contribution with Dim 1 and Dim 2 (Fig. 7). This result is due to the TSS concentration in domestic wastewater during this period.
At the output of the lagoon, the oxygenation condition improved the oxidation of ammonium to nitrate by nitrifying bacteria; however, nitrate can reduce to nitrogen gas through heterotrophic denitrifiers under low oxygenation conditions [Zekker et al. 2019]. Finally, there were significant linear dependencies between NH 4 + , and TP, which can be attributed to the high efficiency of the lagoon unit to reduce nutrients from wastewater, Hence, the algae can assimilate nutrients from wastewater into their biomass under photosyn-

The proposed complementary treatment
The WWTP treats wastewater up to the secondary level; then the purified wastewater is discharged at a Oued. Along this Oued, this water can be reused anarchically by farmers, who can pump it from the river to irrigate their fields. Therefore, the quality of the water currently produced requires additional treatment for reuse in the irrigation of category crops, called "C", i.e., cereals, fodder, pastures and tree plantations. Just downstream of the pressurizing station a collective filtration station will be set up to further reduce organic matter and the installation of an UV disinfection device to ensure quality water for use in drip irrigation notably bacteriological parameters.

CONCLUSIONS
The performance efficiency of the lagoon natural for treating urban domestic wastewater was evaluated in a small urban community for five years under arid climatic conditions. The findings of this study have drawn the following conclusions: 1. Effectiveness of the lagoon system in the removal of organic matter, nutrient, and fecal contamination was; 37% of TSS, 63% of BOD 5 , 60% of COD, 61% of NH 4 + , 37% of TP, and 6.5 log units of coliforms during four years of monitoring. 2. Season period significantly (p < 0.05) affected the removal of organic matter and nutrients in comparison with fecal coliforms content, which were unaffected by season change. 3. The quality of the treated wastewater does not always comply with the Moroccan norms of reuse of the effluents. Indeed, additional treatment is proposed for the reuse of wastewater treated by this system, consisting of a collective filtration station and a UV disinfection device.