Introduction

Water demand is increasing at a higher rate than population growth, whereas water availability is declining due to growing competing demands from various users [1, 2]. For instance, it is projected that by 2030 the world will face a 40% water deficit. And this water scarcity is estimated to seriously affect the livelihood of one-third of the world’s population. Global demands on food are expected to grow by 40% by 2030 while annual grain losses due to water scarcity are anticipated to be 30% [3, 4]. Consequently, water security is considered to be among the top five global risks in terms of development impact [5]. In this regard, Ethiopia is considered as a water stressed country with renewable water supplies below 1700 m3 per capita per year and with an economic water scarcity [6]. Ethiopia is also listed among the countries that are expected to fall below the water scarcity level (1000 m3 capita−1 yr.−1) by 2030 according to the populations and renewable water resources projections [7].

Intensified abstraction of available water resources through expansion of water supply infrastructures, strategic planning for virtual water [7, 8], improving water productivity [9], and development of non-conventional sources [10] are some of the current strategies to increase water supplies. Nevertheless, efforts to improve wastewater management are generally inadequate. As a result, about 75–90% of the supplied water is being discharged as untreated wastewater [11, 12]. On the other hand, wastewater is emerging as an affordable and reliable alternative water supply for urban and outskirts to support their economy, particularly in arid and semi-arid regions [13]. In the Ethiopian context, rapid urbanization, scarcity of freshwater, lack of wastewater facilities, and increased demands for agricultural products coupled with an absence of monitoring and enforcement mechanisms make wastewater management complex. The recent ambitious expansion of public universities to currently 35 with a collective enrolling capacity of 600,000 students in the regular program with 11 new ones are in the pipeline and their inclusion in the urban centers where wastewater treatment facilities do not exist are leading to increased point sources of untreated wastewater. Consequently, application of untreated wastewater for various uses, predominantly extensive irrigation, is a common practice.

The prevalent irrigated products are vegetables because of their short marketing chains to major urban markets. In fact, continuous use of untreated wastewater for irrigation carries major public health and environmental risks [14]. One of the major concerns is the accumulation of heavy metals in the irrigated soils and crops in the food chain from untreated wastewater. Although crops/vegetables require small amounts of essential trace elements to grow because of their heavy metal uptake/absorption, translocation from the root zone into the plant tissue and subsequent accumulation capabilities may contain a higher amount of these metals in their body, including Ni and Cd which have no known functions but may cause potential health risk through the food chain [15]. Therefore, integrated stepwise management approaches to regulate wastewater use to ensure adequate safeguards and maximize its contribution to sustainable livelihood development are critically important. Nevertheless, the strength and heavy metal contamination of domestic (university sources) wastewater are not well studied and inconsistent conclusions arising from the limited studies attributed to its heterogeneity influenced by location and household characteristics [16]. This remains a problem, particularly in developing countries like Ethiopia where laws restricting exposure to wastewater do not exist. However, quite many research reports are available on the accumulation of heavy metals in soil and subsequent translocation and bioaccumulation by crops grown in wastewater irrigated soils [17, 18]. Furthermore, gaps exist in evidence-based law-making measures committed to wastewater management in Ethiopia, which hinders attempts by the regulators and policy makers to influence common problems that occur at local and national levels through established rules and processes. Hence, this research aims at determination of the seasonal variabilities of strength, salinity, and heavy metal contents of wastewater generated from a public university, mainly organics, nutrients, and metals (Cd, Cr, Cu, Pb, Ni, Zn, Fe, Mn, and Co). In addition, a brief insight is provided about legislative requirements in Ethiopia to ensure the provision of sustainable livelihood in relation to emerging agricultural practices in the urban settings.

Materials and Methods

Study Site Description

The study was conducted in Mekelle University (MU), located in Mekelle city, the capital of Tigray National Regional State situated in the northern Ethiopia at 39° 28′ 60″ E and 13° 28′ 43″ N. The city covers 64,476 ha and is inhabited by more than 323,700 people with a population density of 502.1 persons per square kilometer and a mean annual population increase of 4.7% [19]. The area is largely hot, tropical semiarid and is vulnerable to climate change, with highly erratic and unreliable unimodal rainfall between June and September and frequent occurrence of drought with mean monthly precipitation and temperature of 51.67 mm and 18 °C, respectively [20]. MU is one of the old public higher learning institutions in Ethiopia with 31,872 students enrolled in nine institutes, seven colleges and 23 departments organized in six campuses located in different parts of the city (Fig. 1).

Fig. 1
figure 1

Locations of Mekelle University campuses

Full size image

Wastewater Sampling, Preparation, and Analysis

Composite samples of untreated wastewater were collected from holding tanks located in Endayesus (main) campus. The tanks are connected to dormitories equipped with lavatory and manual laundry facilities, cafeterias, beauty salon centers, and laboratory facilities. These tanks are subjected to continual daily desludging by heavy-duty vacuum trucks. The samples were collected from the trucks at different intervals during loading. Samples were collected in 2015 during dry and rainy (wet) seasons of April and August, respectively. All samples were collected and prepared according to standard procedures described by APHA [21]. The concentrations of Pb, Cu, Fe, Cr, Co, Cd, Ni, Mn and Zn in the wastewater samples were determined using fast sequential atomic absorption spectrometer (Varian AA240FS). Portable multifunction pH, electrical conductivity (EC)/total dissolved solids (TDS) meter electrode, and probe (Wagtech pH/mV/°C Meter, Hand Held, CyberScan pH11) were used to measure pH, EC, and TDs values of the samples. Sample preparation and analysis were performed at the Ezana Analytical Laboratory in Mekelle. In addition, 5-day biochemical oxygen demand (BOD5) and chemical oxygen demand (COD) were determined using the 5-day BOD test at 20 °C and open reflux method, respectively. Concentrations of total nitrogen, phosphate, total phosphorus, and sulfate were determined using multiparameter photometer (Palintest wagtech Photometer 7100 model). Each equipment was calibrated before use with standard procedures. Reagent blanks and duplicate samples were performed in parallel for each analysis. Besides, standard samples of known concentration were incorporated within each batch of samples analyzed for heavy metals. Statistical difference between the values of the samples collected during dry and rain (wet) seasons was performed using independent t test. Pearson correlation analysis was also performed to test the relationship among the analyzed metals. All statistical analyses were performed using Origin 2015 graphing and analysis software, a statistical significant difference was considered for value p < 0.05.

Results and Discussion

Wastewater Sources

The daily wastewater generation of Mekelle University ranged from 827 to 3478 m3 (with 1687 m3 mean value) and daily per capita generation of 65.27 L, considering 25,844 regular students enrolled in the year. The capita wastewater generation is very low compared to the international estimations of 320 L given by Metcalf and Eddy [12]. It is mainly attributed to the deficit in water supply which is a serious situation in the city of Mekelle. Wastewater management technique in all Mekelle University campuses is a localized septic tank system and centralized holding tanks with continual desludging, except at the college of health sciences (Ayder campus), which has a wastewater stabilization pond. However, according to Haddis et al. [22], because of faulty design and management, the stabilization pond is not effective. Buildings and facility centers in the remaining campuses of the University are connected to either individual or communal tanks. About 96 m3 of sewage per day, which is 5.7% of the generation, is being collected using heavy duty vacuum trucks. The counter balance wastewater either overflows downstream or percolates to the ground from the septic tanks. The localities where the septic tanks overflows are corners of an unpleasant environment and sources of pollution (soil, water, odor, esthetic) putting health danger to the university community and the nearby communities.

Public universities have high water demands and are sources of ever increasing quantities of wastewaters. This wastewater can be a reliable resource for the enhancement of agricultural productivity (source of nutrients and water); if properly planned, particularly in areas where water is scarce like the study area of this research. In this regard, wastewaters most importantly from higher learning institutions located in semiarid regions, like Mekelle university can be considered as a point source discharged in areas where higher water demands and potential opportunities for reuses exist. Untreated wastewaters have been disposed into nearby natural streams, resulting in degradation of fresh water and biota. The farmers at downstream communities use the water for irrigation of crops with high market values, particularly vegetables (for both market and home consumptions) which may be consumed raw, similar to the situations in other developing countries [23]. In Mekelle city, directly or indirectly irrigated lands using wastewater and the number of beneficiary household are increased annually. Irrigated land within the city dedicated to vegetables was about 107.3, 122.0, and 175.0 ha in 2013, 2014, 2015, where potential irrigable land of 900 ha is available. There has been a significant increase in the number of beneficiary households within the city participating in irrigation, from 2063 in 2012 to 2860 households in 2014. Land application of raw sewage from vacuum trucks by spreading over farmlands to utilize available nutrients as a soil conditioning is another route for wastewater disposal. Irrigation is based on traditional infrastructures and farms commonly use furrow and flood irrigation techniques. This is an unsafe practice for the producers due to exposure to untreated wastewater and low level of understanding about health consequences [24]. Disregard of this health risk may impact the livelihood of the producers and peoples involved in the supply chain and consumers. With proper treatment and management, the wastewater from the university can be a valuable resource to improve the livelihood of thousands of people in a sustainable way. However, this needs a proper understanding of the nature of the wastewater and the type and degree of treatment required, particularly of pollutants with potentially serious public health risks, including heavy metals.

Salinity and Heavy Metal Contents of the Wastewater

Concentrations of the analyzed parameters in the wastewater samples and the corresponding threshold limits are presented in Table 1. Significant contents of heavy metals were found in this work, similar findings were reported from previous researches [11, 16, 25]. Results of independent t test analysis revealed statistically significant differences between the mean concentrations of Co, Zn, Cr, Ni, Fe, Mn, and EC between the dry and rainy seasons (p < 0.05). In contrast, pH and the contents TDS, Cu, Cd, and Pb were comparable in both seasons. The primary reason for these variations may be the shortage in water supplies in the dry season and the effect of dilution during the wet season. As shown in Table 2, significant positive associations were found between Zn with Mn, Fe, Cr, and EC. Likewise, Fe has shown a strong positive correlation with Mn and EC. The associations between the parameters were in the order of Fe/Mn > Zn/Fe > Zn/Mn > Fe/EC > Mn/EC > Zn/EC > Zn/Cr. The results also showed moderately positive relationships between concentrations of Cr with Fe and Mn and negative associations between Pb and Cd. On the other hand, the associations of Co with Cd and Cu were not significant with zero coefficient value. Positive correlation of species suggested that same source of origin [26].

Table 1 Characteristics and seasonal variability of the wastewater
Full size table
Table 2 Pearson correlations matrix of metals and EC
Full size table

The results of the study were compared with available literature; in fact, heterogeneity exists among domestic wastewaters ascribed to the contributing activities, climate, water supply conditions, and the socioeconomic classes. For instance, according to WHO [14], chemical concentrations are higher in arid and semiarid areas, because less water is used at the point of generation and considerable evaporations occur during collection. Household activities, personal care, and laundry products are some of the principal sources of cadmium in domestic wastewater, as reported by Tjandraatmadja et al. [25]. These are also common activities in the study case. Similarly, phosphate based softening agents are major sources of cadmium [27]. Cadmium concentration was found threefold (0.03 mgL−1) higher than the threshold limit (0.01 mgL−1). This finding is comparable with the findings of Qadir et al. [28] in municipal wastewater but greater than the values in gray water reported by Revitt et al. [29]. Cd, which can be absorbed by food plants/crops, is toxic to humans. For instance, vegetables contribute 70% of Cd intake in humans [28].

Compared to the values reported Pescod [30], slightly lower concentrations of Cu and Zn and greater Mn were found in the present study. The higher values for Mn can be attributed to the content of the metal in the water supply. Similarly, hardness, the very nature of the water supply in Mekelle has reportedly contributed to heavy metals in domestic wastewater, in general, and copper, in particular. For instance, Comber and Gunn [27], estimated hard water can be responsible for 64% Cu of which 93% are from plumbing interactions. Additionally, household products are known to contribute small amounts of copper according to Tjandraatmadja et al. [25]. Gray water which comprises water from kitchen, sink, laundry, bathing, and other washing activities is the largest proportion and the main source of heavy metals in domestic wastewater. Hence, a comparative discussion on the concentrations of metals in gray water and domestic wastewater is indispensable. Galvanized iron pipe and fittings, body care products, washing agents, and water supply were also reported to be sources of zinc [27]. Results of zinc concentrations were comparable to the values reported by Yost et al. [31].

The major Fe contributing materials according to Tjandraatmadja et al. [25] are personal-care products, washing agents, toilet freshener, toilet paper and toothpaste, which are common in the study area. Results of iron concentration in this study were higher than the values reported by Yost et al. [31]. The local water supply has also been reported for its contribution to the concentrations of Fe, Cu, Pb, Cr, and other metals. In this regard, the concentrations of Fe, Cu, and Mn in the tap water during the study were 270, 420, and 320 μgL−1, respectively. Similarly, previous investigations on heavy metal contents of the water supply in the study area such as Mebrahtu and Zerabruk [32] have reported the concentrations of Fe, Zn, Pb, and Cr with range values of 97–919, 80–583, 69–106, and 131–158 μgL−1, respectively. Household appliances such as washing machines that use dishwashers solution and fabric dye are commonly used at Mekelle University and may be the main contributors of Cr in domestic wastewater [33]. The results obtained for Cr were comparable with values reported by Yost et al. [31]. Similarly, household appliances, including washing machine and dishwasher, sanitary installations and feces (accounting about 60%), household products like shampoo, laundry, and other personal and equipment washing agent, and hair conditioners, all used in the study area were reported as major contributors of nickel in domestic wastewater [25, 27, 33]. While comparable nickel concentrations were reported by Yost et al. [31], slightly higher values were observed in the present study compared with the literature for gray water [29].

A study conducted by Tjandraatmadja et al. [25] in Australia reported that toilet paper, air fresheners, personal care, and dishwashing products were the principal contributors of Pb in streams polluted by domestic wastewater. In this connection, Tjandraatmadja and Diaper [16] reported that about 30–80% source of Pb was from laundry and bathrooms. Lead concentration in this study was lower than the concentrations reported by Yost et al. [31] but in line with Tjandraatmadja and Diaper [16]. In contrast, the concentration of lead in the current study was slightly higher than the values reported for gray water [29]. Deterioration of sewerage pipelines and the wear of household plumbing fixtures are considered important sources of Co in domestic wastewater [23, 34]. Concentrations of Co, Cd, Fe, Mn and to some extent Cu in the present study were higher than the allowable limit for irrigation. While the concentration of Pb, Zn, Ni, and Cr are below threshold levels. In agriculture, these threshold concentration levels depend highly on the irrigation practices. Thus, application of threshold limits requires adjusting based on the irrigation practices such as the quantity of water used, irrigation technique and crop type [30]. Mean pH values measured during both seasons were not significantly different and within the recommended limits for irrigation.

The salinity of the wastewater measured in terms of EC and TDS had slightly lower EC and comparable TDS values as those reported by Tjandraatmadja et al. [25] but higher value of EC was observed compared to the findings of Ren et al. [35]. However, the values were above the severe restriction limit values of 3 dSm−1 and 2000 mgL−1 for EC and TDS, respectively. As shown in Table 1, the values determined for COD and sulfate in the rain season and dry season were not statistically different (P < 0.05), however the concentration of BOD5, TN, TP, TKN, and phosphate showed significant variation. While the concentrations of TN in the present study were greater than the contents reported by Bilgin et al. [36], comparable values for COD, lower values for TN and BOD5, and higher TP were observed compared to Henze and Comeau [37]. In contrast, COD and BOD5 in the present study were found higher than the values in the literature for domestic wastewaters [35, 38]. Variations are expected because the composition of domestic wastewaters has large and dynamic variations in terms of both place and time depending on the source, water supply, climate, and the lifestyles of the contributing communities and sewage components [37].

Understanding of the composition and strength of the wastewater is quite important in determining the degree of treatability and corresponding treatment methods, potential intervention mechanisms and possible reuse options. In this regard, knowledge of the organic contents and the proportion of the readily degradable and non-biodegradable are fundamentally crucial. The organic content of a wastewater is expressed in terms of the biochemical oxygen demand (BOD) and the chemical oxygen demand (COD) [39]. While the values of EC and pH in the present study are comparable with the values reported by Pérez-Marín et al. [40], higher values of BOD5, COD, TP, total Kjeldahl nitrogen (TKN), and phosphates were found compared to the values from Universities in Spain [40] and in Greece [41]. Hence, the wastewater under the current study can be classified as strong when evaluated in terms of TP, TN, TKN, chemical and biochemical oxygen demands (COD and BOD5), EC, and TDS [12, 39–41]. This may be attributed to the deficit in water supply and activities contributing concentrated wastewater such as cafeterias, kitchens, dishwashing, beauty salons and laboratories.

In spite of the higher COD and BOD5 concentrations observed in this study, the COD/BOD5 ratio was comparable with the values reported by Jamrah [39]. Furthermore, investigation of the pH values revealed that the wastewater was with the optimum range of 6.5 to 7.5 for bacterial growth [42] and within the range of favorable range for most biological life (4.0 to 9.0) [39]. The COD/BOD5 ratio in the dry season (4) was higher than the value in the wet season (3) which are slightly higher than the values reported by Pérez-Marín et al. [40], but comparable to the findings of Jamrah [39]. The ratios were higher than the typical range of 1.25 to 2.5 and limit value of 1.6 for domestic wastewater, it can be concluded that the wastewater is categorized as containing higher proportions of the nondegradable organic matter [12, 39, 43]. The ratio of BOD5/COD has been used as an acceptable indicator to evaluate the biodegradability capacity of a wastewater with a frequent value ranged from 0.3 to 0.8. Accordingly, for easily treatable wastewaters, the ratio is greater than 0.5 and wastewaters with a value less than 0.5 require pre-treatments prior to biological treatment, but it would be difficult to treat biologically when the ratio falls below 0.3 [12, 44, 45]. The BOD5/COD ratios of the wastewater under the present study were 0.33 and 0.25 in the wet and dry season, respectively, which are slightly below the value for effective biological treatment.

The ratios of COD: TN: TP of the studied wastewater were 100:2.8:3.8 and 100:6.2:2.5 in the wet and dry seasons, respectively, that revealed deficiencies of TN and fluctuations in nutrient contents when compared with 100:5:1 for domestic wastewaters [42, 46]. Hence, both the relatively high values of nonbiodegradable organic matters and the nutrient imbalance might limit the effectiveness of biological treatments. The higher nonbiodegradable organics and nutrient imbalances may be attributed to the presence of the gray water originating from kitchen sinks, cafeterias, dishwashers, laboratories and beauty salons. The ratio of BOD5 to TN has also been used as an index for nitrification and denitrification processes responsible for the removal of nitrogen compounds from wastewater [47]. Accordingly, if the BOD5/TN ratio is less than 4, nitrification is a dominant process, but a combined process for higher values. The ratios were 12.1 in the wet and 4.1 in the dry season, where both process are expected.

Confronting Realities

Use of wastewater in agriculture, a widespread practice worldwide [13] has the potential for both positive and negative impacts on crop production, public health, and ecology [48], with enormous impacts on the lives of poor communities [49]. Significant volumes of untreated wastewaters are becoming available to local farmers in areas where no other alternatives exist. Constrained by the lack of awareness on the associated health hazards and absence of protective safety materials, farmers engage in unsafe traditional irrigation practices with potential health risks to themselves; their families, and others in the food supply (chain). The situation is even more critical in countries like Ethiopia, where laws restricting exposure to heavy metals do not exist. Moreover, wastewater treatment facilities neglect heavy metal contamination in the designing, operation, and disposal of effluents from domestic sources. The market values and competitive advantage of proximity to potential markets are the primary reasons for the continued use of wastewater for irrigation. The externalities of wastewater generators in terms of pollution are cyclical, where the urban centers discharge untreated wastewater and the downstream farmers return unsafe crops to the urban markets.

In this regard, urban centers in developing countries are becoming megacities requiring megawater, generating megasewage, and creating megaproblems [8]. On the other hand, crop production using wastewaters will be a competent option for the poor people who have farm lands in the urban and peri-urban areas. Planned utilization of wastewater will have a broader array of socioeconomic and ecological benefits for the developing countries. To illustrate this, use of wastewater for irrigation helps with nutrient recycling and associated reduction in artificial fertilizers. According to WHO [14], at an irrigation rate of 1.5 m/year (1.5 m3 of irrigation water per square meter of field area per annum), treated municipal wastewater can supply 225 kg of nitrogen and 45 kg of phosphorus per hectare yearly. It was also indicated as a means to combat malnutrition and to improve food security [50]. If wastewater use is integrated with long-term water resources planning, it could be a reliable supply of water in areas with limited alternative water sources [15]. For instance, a city with a population of 500,000 would generate enough wastewater to irrigate approximately 6000 ha [30]. Furthermore, it improves urban-rural linkage in water and crop supplies and can mitigate downstream ecological degradations. Hence, countries like Ethiopia need to develop policy framework to regulate wastewater-related use, disposal, and recycling practices to solve water, food, and environmental health challenges and harness potential opportunities for sustainable development.

Policy and Legal Frameworks for Wastewater Reuse for Irrigation

Environmental issues in Ethiopia have become the government’s agenda following the 1992 UN Conference on Environment and Development at the Earth Summit. The mechanisms of environmental protection adopted by Ethiopia since then are characterized as a three-stage approach. This comprises the incorporation of environmental rights under the constitution of Ethiopia (1994), the establishment of an environmental policy of Ethiopia (1997), and other national and sectoral policies (Table 3). As a result, significant improvements heading towards achieving sustainable development have been recorded. A scarcity of fresh water, relative availability, and productivity of wastewaters and the growing demand for agricultural products are some of the drivers that encourage wastewater reuse. Nonetheless, the concern of wastewater reuses for irrigation, particularly for the production of human consumable crops has remained an urgent and growing concern.

Table 3 Ethiopian environmental legal frameworks pertaining to wastewater
Full size table

The agricultural use of wastewater has been regulated in some countries. As a result, there are guidelines on wastewater reuse for irrigation [14, 15, 30]. However, in many developing countries, these guidelines are not followed [7]. For instance, the legislative frameworks in Ethiopia applicable to wastewater management summarized in Table 3, were developed to protect the natural environment and public health. Nevertheless, none of them address the issue of reuse of wastewater for the irrigation of human consumable crops and the degree of treatment required and monitoring for domestic wastewater in terms of heavy metals. These omissions are due largely to the absence of specific effluent standards for point sources of domestic wastewater, including public institutions. The enforcement and monitoring responsibilities also differ among different organizations, including the Ministry of Health, Ministry of Environment and Forest, and the Ministry of Water and Energy. Moreover, there is a lack of monitoring limit values on the heavy metal contents of wastewater to be used for irrigation of crops and types of crops permitted to grow on soils treated with wastewater containing elevated heavy metal contents. The conditions of exposure of peoples in the supply chain also need to be considered. Consequently, wastewater treatment facilities are designed without due attention to the removal of these metals and poorly treated/untreated wastewater from various sources are being used for the irrigation of crops for human consumption. Time has come for the sustainable management and development of appropriate guidelines and regulation of wastewater uses in Ethiopia by setting threshold limiting values for each metal, minimize human exposure in farming and vendors of produces, specify techniques, and restriction of crops to be grown in soils irrigated with wastewaters to protect the producers and consumers. Furthermore, it is vital to consider the wastewater use strategy in long-term water resources plans to improve the productivity and enhance ecological protection. With proper treatment and monitoring, wastewater will be a huge opportunity to improve the livelihood of thousands of people in the urban and peri-urban areas where fresh waters are scarce. A stand-alone decentralized constructed wetland treatment system or hybrid systems featuring sand filtration are potential treatment options.

Conclusion

Application of wastewater with questionable quality for irrigation to produce food crops has been a common practice in developing countries, including Ethiopia, contributing to the livelihood of thousands of people in the supply chain. Analysis of the wastewater of Mekelle University indicated that the concentrations of organics, nutrients, Co, Cd, Fe and Mn are greater than allowable limits in irrigation water. Similarly, salinity measured in terms of EC and TDS is higher than the severe restriction limit values. However, the concentrations of Pb, Zn, Cu, Ni, and Cr were slightly below the recommended threshold levels. Thus, considering the levels of heavy metal contents emitted under the current practices, it may be unsafe to use untreated domestic wastewaters, particularly from university sources for irrigation of food crops. Hence, water resources planning in Ethiopia and probably other developing countries need to seriously consider treatment options prior to wastewater reuse for agricultural irrigation. Effective legal framework to regulate the practice and prevent exposure of producers and consumers to heavy metals should be put in place. Restriction of the types of crops, irrigation techniques, and monitoring of metal accumulation in soils and crops are extremely important to protect consumers and livelihoods of farmers from harmful effects of wastewater reuses.