3.1. Physical and Chemical Characteristics of Municipal Wastewater
According to the results of municipal wastewater analysis value of pH at initial and final irrigation points were 8.14 and 8.26 (Table 1). The total dissolved solid (TDS) municipal wastewater varied in the range 762.54 to 945.32 mg L− 1 from final to initial point. The BOD and COD values in the study sites were varied from 167.0mg/l to 207.3mg/l and 246.43mg/l to 286 mg/l respectively (Table 1), Concentration of sodium in municipal wastewater varied in the range of 33.3 to 44.5 mg L− 1 at the final to initial discharge points (Table 1). The initial and final point concentration of ammonium-nitrogen and nitrate-nitrogen were 7.56 to 14.25 mg/l and 9.43 to 11.60 mg/l respectively (Table 1).
Table 1
Characteristics of municipal wastewater quality
Descriptive Statistics |
Parameters | Initial point | Final point | Mean | SD(+) | FAO/WHO Perm. Limits |
pH | 8.26 | 8.14 | 8.2 | 0.085 | 6.5–8.4 |
EC, dS/m | 2.34 | 2.32 | 2.33 | 0.014 | 0.7-3.0 |
TDS, mg/l | 945.32 | 762.54 | 853.93 | 129.24 | 450–2000 |
TSS, mg/l | 67.98 | 49.65 | 58.82 | 12.96 | 50 |
DO, mg/l | 0.0045 | 0.0003 | 0.002 | 0.002 | 5–6 |
BOD, mg/l | 207.3 | 167.0 | 185.15 | 28.49 | 60 |
COD, mg/l | 288 | 246.43 | 266.22 | 27.98 | 200 |
Na+, mg/l | 44.5 | 33.3 | 38.9 | 7.92 | 200 |
Ca2+, mg/l | 74.8 | 64.2 | 69.5 | 7.49 | 400 |
Mg2+, mg/l | 54.3 | 49.6 | 51.95 | 3.32 | 60 |
K+, mg/l | 66.33 | 51.44 | 58.84 | 10.46 | < 10 |
HCO3−, mg/l | 40.5 | 35.3 | 37.9 | 3.676 | 1.5–8.5 |
CO32−, mg/l | 24.5 | 19.4 | 21.95 | 3.606 | < 10 |
NH4−N, mg/l | 14.25 | 7.56 | 10.91 | 4.73 | - |
NO3-N, mg/l | 11.60 | 9.43 | 10.52 | 1.53 | 5–30 |
TN, mg/l | 29.6 | 24.78 | 27.19 | 3.408 | 5–30 |
P, mg/l | 9.43 | 8.64 | 9.04 | 0.56 | < 10 |
SAR | 4.988 | 4.414 | 4.701 | 0.794 | 3–9 |
ESP % | 6.538 | 5.538 | 5.763 | 1.096 | - |
DO = dissolved oxygen, TDS = total dissolved solid, TSS = total suspended solid, EC = electrical conductivity, BOD = biological oxygen demand, COD = chemical oxygen demand, per.limit = permissible limits, SAR = sodium adsorption ratio and ESP = exchangeable sodium percentage |
The mean ion concentrations in wastewater at both sample collection sites were compared by t-tests at a confidence level of 95%. Among the two sites, wastewater samples taken from the initial points and final points showed significant differences (p ≤ 0.05), and this variation might be due to natural processes of water.
3.2. Impacts of Municipal Wastewater on Soil Physical Properties
At wastewater irrigation sites, there were no significant differences in soil moisture under MWWI < 10 and MWWI > 10. Both nonirrigated and irrigated farmland showed significant differences at the p < 0.05 level in the soil moisture results (Table 2). This variation might be due to the higher contents of organic matter generated from municipal wastewater. The presented result was in agreement with [12, 13 & 14] variation in bulk density, total porosity and soil moisture contents between the irrigated and control sites, which might be due to the addition of organic matter in irrigated farmland (Table 2). In other studies, the moisture contents of the wastewater irrigated plots were consistently higher than those of the control [15].
Clay contents were highly and significantly different between the nonirrigated and irrigated farmland. The highest clay content (26%) was recorded for nonirrigated farmland, whereas the lowest value (23%) was recorded for wastewater-irrigated farmland (Table 2). On the other hand, the lowest clay content was recorded at the farmland irrigated for more than ten years. The dominant soil particle in the study sites was sand followed by clay and silt. Silt content was the least at all sites.
Table 2
Selected physical properties of the soil at the study sites
Descriptive Statics | | PSD % | | BD (g/cm3) | TP % | Moisture % |
Sand | Silt | Clay |
Control | 56 | 18 | 26a | 1.56a | 41.1C | 10.86b |
MWWI < 10 | 61 | 15 | 24b | 1.28b | 51.7b | 24.98a |
MWWI < 10 | 58 | 19 | 23c | 1.15b | 56.6a | 26.2a |
Mean | 58.33 | 17.3 | 24.33 | 1.33 | 46.4 | 20.68 |
SD (±) | 2.52 | 2.08 | 1.53 | 0.21 | 5.3 | 8.53 |
SL | NS | NS | *** | *** | *** | * |
MWWI > 10 = municipal wastewater irrigated for more than ten years, MWWI < 10 = brewery, municipal wastewater irrigated for ten years, PSD = particle size distribution, SL = significance level NS = not significant; *=significance at p < 0.05; **=significance at p < 0.01; ***=significance at the p < 0.001 level. |
3.3. Impacts of Municipal Wastewater on Soil Chemical Properties
The results in Table 3 indicated that all soil chemical properties were slightly affected by the wastewater. At the study sites, the soil reaction was slightly alkaline in municipal wastewater-irrigated farmland. A higher pH value (7.74) was recorded with nonirrigated (control) farmland, and the lowest pH value (7.34) was recorded with irrigated farmland with municipal wastewater for less than ten years (Table 3). The variation in soil pH might be due to the presence of a high content of relatively ammonium in the wastewater (Table 1), resulting in its accumulation in the soil. This suggestion was supported by the finding of [16] that soil pH decreased with wastewater irrigation due to the oxidation of organic compounds and nitrification of ammonium. Nitrification of this ammonium would serve as a source of hydrogen ions, which may lead to a decrease in the soil pH [13].
The total soluble salt content expressed as electrical conductivity (EC) is an important indicator of soil health. It affects crop yields, plant nutrient availability, and the activity of soil microorganisms, which influence key soil processes [17]. The electrical conductivity of the soils was significantly influenced by the wastewater. The highest electrical conductivity (0.0328 dS/m) was recorded for municipal wastewater for more than ten years (MWWI > 10) of irrigation, whereas the lowest value of EC was recorded for the nonirrigated site (0.062 dS/m) (Table 3). At nonirrigated and irrigated farmland, the differences in soil electrical conductivity were significant (P < 0.001).
Wastewater irrigated farmland; soil electrical conductivity increased with increasing years of wastewater application. Additionally, the soil EC significantly increased at MWWI < 10 than at the MWWI > 10 sites. The increase in EC (electrical conductivity) of the soil due to irrigation with wastewater is mainly attributed to the high level of TDS (total dissolved solids) of the wastewater that would accumulate in the soil with continuous wastewater application. This result is in harmony [18 & 19].
Table 3
pH, EC (dS/m), and heavy metal (mg/kg) concentration of soil at different irrigation sites
Descriptive statistics | | | Parameter | | | | |
| pH | EC | Cu | Zn | Cd | Pb | Cr |
Control | 7.74 | 0.062c | 5.6 | 6.85 | 1.67c | 0.29b | 0.54 |
MWWI < 10 | 7.34 | 0.230b | 3.2 | 5.06 | 2.5b | 0.57a | 0.36 |
MWWI > 10 | 7.68 | 0.328a | 4.53 | 8.43 | 3.33a | 0.57a | 0.54 |
SD (±) | 0.22 | 0.13 | 1.20 | 1.67 | 0.83 | 0.16 | 0.10 |
SL | NS | *** | NS | NS | *** | * | NS |
FAO Perm. Limits | 6.5 | < 10 | 1–12 | 12–60 | 0.02–0.5 | 0.3–10 | 0.002-0.2 |
MWWI > 10 = municipal wastewater irrigated for more than ten years, MWWI < 10 = municipal wastewater irrigation for 10 years, SL = significance level NS = not significant; *=significant at p < 0.05; **=significant at p < 0.01; ***=significant at the p < 0.001 level. |
3.4. Total heavy metals
The concentrations of Zn, Cu, Cd, Pb and Cr at MWWI > 10 (municipal wastewater irrigated land for more than ten years) were 8.43, 4.53, 3.33, 0.57 and 0.54 mg/kg, respectively (Fig. 2). However, at the nonirrigated site, the recorded results were 6.85, 5.6, 1.67, 0.29 and 0.54 mg/kg for Zn, Cu, Cd, Pb and Cr, respectively. The concentrations of Cd and Pb were significantly higher at irrigated sites than in nonirrigated areas. This might be due to the release of these elements by the fermentation process and their entry into the soil with irrigation water. According to [20 & 21]. A large volume of waste water generation that is concentrated in metals is high in city, but use of treated waste water irrigation is low in developing countries.
3.5. Organic matter (OM) and soil organic carbon (SOC)
According to [22], organic matter is widely regarded as a vital component of soil fertility because of its role in physical, chemical and biological processes to supply plants with nutrients and to help soil maintain moisture. At wastewater irrigation sites, the organic matter content (OM) was positively and significantly influenced by wastewater. The highest organic matter (OM) content (2.69%) was observed for the soil irrigated for more than ten years, whereas the lowest value of OM was recorded for the nonirrigated sites (Table 4). This might be due to the release of organic compounds by the brewing process and their entry into the soil with irrigation water. The present study was consistent with the finding [23], who reported that the soil organic matter content (OM) increased with wastewater irrigation application and depended on the period of application. According to [24], the organic matter content in the soil also increased as the number of irrigations increased, showing a benefit to the soil. The organic carbon contents of the soil were not significantly influenced by wastewater irrigation (Table 4). Soil organic carbon (SOC) is the most important indicator of soil quality and plays a major role in nutrient cycling [25].
Table 4
SOM (%), SOC (%), AVP (mg/kg) and TN (%) concentrations at different sites.
Descriptive Statistics | | Parameters | | |
| SOM | SOC | AVP | TN |
Control | 1.35c | 0.999 | 9.71c | 0.19 |
MWWI < 10 | 1.72b | 0.781 | 11.27b | 0.20 |
MWWI > 10 | 2.69a | 1.562 | 13.10a | 0.21 |
SD (±) | 0.60 | 0.40 | 1.69 | 0.005 |
SL | ** | NS | *** | NS |
SOM = soil organic matter, SOC = soil organic carbon, AVP = available phosphorus, TN = total nitrogen SL = significant level NS = not significant; *=significant at p < 0.05; **=significant at p < 0.01 and ***=significant at p < 0.001 level. |
3.6. Concentration of different MWWI Chemicals
The soil AVP was significantly different (p < 0.001) in nonirrigated (control), municipal wastewater irrigation farmland for less than ten years (MWWI < 10) and municipal wastewater irrigated farmland for more than ten years (MWWI > 10).
The CEC was significantly different along the nonirrigated and irrigated farmland, as shown in Fig. 3. The highest CEC value was recorded in municipal wastewater-irrigated farmland for more than ten years, and the lowest value was observed in nonirrigated land. CEC was significantly higher in MWWI > 10 than in MWWI < 10 and nonirrigated (control) plants.
3.7. Pearson’s Correlation between Chemical Properties of Wastewater and
Wastewater Irrigated Soils
The relationships between different chemical properties and ion concentrations of wastewater and wastewater irrigated soils were analyzed by Pearson’s correlation coefficient. A high correlation coefficient (near + 1 or -1) means a good relation between two variables, and zero means no relationship between them at a significance level of 0.05%. r > 0.7 indicates a strong correlation, whereas the r value is between 0.5 and 0.7, and it shows a moderate correlation between the two parameters.
Table 5. Pearson’s correlation between chemical properties of wastewater and soils of different sites Wastewater parameters
| PH | EC | Ca | Mg | Na | K | TN | AVP | SAR | ESP |
PH | 1 | | | | | | | | | |
EC | 0.994* | 1 | | | | | | | | |
Ca | 0.963 | 0.927 | 1 | | | | | | | |
Mg | 0.999** | 0.997* | 0.952 | 1 | | | | | | |
Na | 0.930 | 0.965 | 0.795 | 0.943 | 1 | | | | | |
K | 0.995* | 0.977 | 0.985 | 0.990* | 0.887 | 1 | | | | |
TN | 0.999** | 0.989 | 0.971 | 0.997* | 0.917 | 0.998 | 1 | | | |
AVP | -0.914 | -0.954 | -0.771 | -0.929 | -0.999** | -0.868 | -0.900 | 1 | | |
SAR | 0.999** | 0.994* | 0.961 | 0.999** | 0.931 | 0.994* | 0.999** | -0.916 | 1 | |
ESP | 0.999** | 0.988 | 0.974 | 0.996* | 0.912 | 0.998** | 0.999** | -0.895 | 0.999** | 1 |
EC = electrical conductivity; TN = total nitrogen; AVP = available phosphorus; SAR = sodium adsorption ratio and ESP = exchangeable sodium percentage. |
** Correlation is significant at 0.001 and * correlation is significant at 0.05 |
The results in Table 5 indicated that there was a significant and positive correlation between pH, Mg, TN, SAR and ESP (P≤0.001) and that these parameters were significantly negatively correlated with available phosphorus and significantly correlated with pH, EC and K+ (p ≤ 0.05).