Seyoum Leta

Phytoavailability of Heavy Metals and Metalloids in Soils Irrigated with Wastewater, Akaki, Ethiopia: A Greenhouse Study

Irrigation with untreated wastewater from several industrial, commercial, and domestic discharges for decades caused accumulation of various heavy metals and metalloids in soils along the Akaki River in Ethiopia. Assessment of environmental threats and the potential phytoremediation of the soils require understanding of the toxic elements’ uptake and distribution in plant parts. Hence, a greenhouse study was performed to examine the phytoavailability and distribution of Cr, Ni, Co, Cu, Zn, Cd, Pb, Hg, Se, V, and As in forage grasses: Oat (Avena sativa), Rhodes grass (Chloris gayana), Setaria (Setaria sphacelata), and the legumes Alfalfa (Medicago sativa) and Desmodium (Desmodium unicinatum). The average contents of Cr, Ni, Co, Cu, Zn, Pb, Hg, Se, and V in the plants were generally higher than the background levels for forage grasses/legumes, and some of these elements were in the phytotoxic range. Root bioconcentration factor (BCF = root to soil concentration ratio) > 1 was observed for Cu (Oat, Rhodes, Desmodium, and Setaria: Fluvisol), Zn (Setaria: Fluvisol), Cd (Rhodes: Fluvisol; Setaria from both soils) and Hg (Oat and Alfalfa: Fluvisol). Alfalfa and Desmodium displayed translocation factor > 1 (TF = shoot to root concentration ratio) for most heavy metals. Most heavy metals/metalloids may pose a health threat to humans and stock via introduction to the food chain. The plant factors (species and plant part), soil factors (soil type, soil fractions, pH, and CEC), and their interactions significantly (p < 0.05) influenced plant heavy metal and metalloid levels. However, the role of plant part and species emerged as the most important on heavy metal uptake, translocation, sequestration, and ultimately transfer to the food chain. Accordingly, the uptake and distribution of heavy metals/metalloids in the plants reflect the potential environmental and health hazards attributable to the use of fodder grasses, legumes, and cultivation of vegetables in soils with polymetallic and metalloid contamination.

Wastewater treatment performance efficiency of constructed wetlands in African countries: a review.

In Africa, different studies have been conducted at different scales to evaluate wastewater treatment efficiency of constructed wetland. This paper aims to review the treatment performance efficiency of constructed wetland used in African countries. In the reviewed papers, the operational parameters, size and type of wetland used and the treatment efficiency are assessed. The results are organized and presented in six tables based on the type of wetland and wastewater used in the study. The results of the review papers indicated that most of the studies were conducted in Tanzania, Egypt and Kenya. In Kenya and Tanzania, different full-scale wetlands are widely used in treating wastewater. Among wetland type, horizontal subsurface flow wetlands were widely studied followed by surface flow and hybrid wetlands. Most of the reported hybrid wetlands were in Kenya. The results of the review papers indicated that wetlands are efficient in removing organic matter (biochemical oxygen demand and chemical oxygen demand) and suspended solids. On the other hand, nutrient removal efficiency appeared to be low.

Presence, Viability and Determinants of Cryptosporidium Oocysts and Giardia Cysts in the Addis Ababa Water Supply and Distribution System

The water supply and distribution system in Addis Ababa, Ethiopia, was investigated for the presence of viable Cryptosporidium and Giardia. A total of 115 samples were collected from water sources, treated water storage tanks, and tap water. Microbial sample collection and analysis were according to U.S. EPA method 1623. The physical and chemical water parameters were also determined in each water sample. Viable Cryptosporidium oocysts and Giardia cysts, collectively referred to as (oo)cysts, were found in two-thirds of the surface raw water samples. Only one of the three raw well water samples had viable oocysts and none had viable cysts. Of the treated water storage tanks, 35% had oocysts (none were viable), and 29% had cysts (only viable in one case). Of the tap water samples 21% had oocysts and 7% had viable oocysts. 1% of the tap samples had cysts, 6% had viable cysts. Turbidity was the only parameter found to correlate with the presence of (oo)cysts. While only one of the treated water storage tanks had viable (oo)cysts, the presence of non-viable (oo)cysts is indicative of problems with the water treatment system. The presence of viable (oo)cysts in the tap water samples showed contamination of treated water during distribution, potentially from underground sewage. There was a statistically significant decrease in free chlorine from the storage tanks to the tap. 22% of the tap samples had no measurable free chlorine. This situation poses health risks to residents of Addis Abba, especially to those with compromised immune system.

Assessment of plants growing on gold mine wastes for their potential to remove heavy metals from contaminated soils

Soils contaminated by heavy metals may pose a threat to human health and the environment. Phytoremediation requires the selection of suitable species. This paper reports a study to assess the capacity of native plants grown on waste dumps and soil affected by tailings material to accumulate and translocate Cr, Ni, Co, Cu, Zn, Cd, and Pb at a gold mine in Ethiopia. Overall, the naturally growing plants, though with no hyperaccumulation, could be very useful for phytoremediation of soils affected by polymetallic pollution.

Post-treatment of tannery wastewater using pilot scale horizontal subsurface flow constructed wetlands (polishing)

In the present study, a pilot scale horizontal subsurface flow constructed wetland (CW) system planted with Phragmites karka; longitudinal profile was studied. The wetland was fed with tannery wastewater, pretreated in a two-stage anaerobic digester followed by a sequence batch reactor. Samples from each CW were taken and analyzed using standard methods. The removal efficiency of the CW system in terms of biological oxygen demand (BOD), chemical oxygen demand (COD), Cr and total coliforms were 91.3%, 90%, 97.3% and 99%, respectively. The removal efficiency for TN, NO3- and NH4+-N were 77.7%, 66.3% and 67.7%, respectively. Similarly, the removal efficiency of SO42-, S2- and total suspended solids (TSS) were 71.8%, 88.7% and 81.2%, respectively. The concentration of COD, BOD, TN, NO3-N, NH4+-N, SO42 and S2- in the final treated effluent were 113.2 ± 52, 56 ± 18, 49.3 ± 13, 22.75 ± 20, 17.1 ± 6.75, 88 ± 120 and 0.4 ± 0.44 mg/L, respectively. Pollutants removal was decreased in the first 12 m and increased along the CW cells. P. karka development in the first cell of CW was poor, small in size and experiencing chlorosis, but clogging was higher in this area due to high organic matter settling, causing a partial surface flow. The performance of the pilot CW as a tertiary treatment showed that the effluent meets the permissible discharge standards.

Two Phase Anaerobic Co Digestion of Tannery Wastewater and Dairy Wastewater: Effect of Operational Parameters on Performance of Hydrolytic–Acidogenic Step

In this study, optimal working conditions for the hydrolytic-acidogenic step in two stageAnaerobic Sequencing Batch Reactor (ASBR) system during anaerobic co digestion of tannery wastewater (TWW) and dairy wastewater (DWW) were investigated. The study was carried out at laboratory scale digesters with 0.4L working volume in semi continuous mode operation under mesophilic temperature (38±2°C). Four mixing ratios of substrates (100:0, 75:25, 50:50, 25:75) at hydraulic retention time (HRT) of 5, 3 and 1days and equivalent organic loading rate (OLR) were examined to assess the effect of each operational parameter on process performance of acidogenic step in the two stageASBR. Based on the result obtained, 50:50 substrate (TWW and DWW) mixing ratio(in terms of COD), 3days HRT and OLR of 3.22gCOD/l/daywere selected as optimum operational conditions for enhanced performance of hydrolytic-acidogenic stage during two stage anaerobic co digestion of the two agro industrial wastes. In addition to this, the degree of acidification was mainly affected by higher OLR and the best result (55.5%) was achieved in the reactor 50:50 operated at 3days HRT and 2.63 gCOD/l/day OLR. In general co digestion of tannery wastewater with dairy wastewater is more important than using tannery wastewater alone for its enhanced performance. This is because of the positive synergistic effects of the co-substrates (dairy wastewater) in terms of well-balanced nutrients, an appropriate C: N ratio and a stable buffering capacity.

Assessment of Heavy Metal Contamination in Vegetables Grown Using Paper Mill Wastewater in Wonji Gefersa, Ethiopia

Heavy metals are among the major contaminants of vegetables. A study was conducted at Wonji Gefersa farms where paper wastewater is used for cultivation of vegetable crops. Four vegetable samples, namely Swiss chard, carrot, tomato, green pepper, as well as paper wastewater were examined for heavy metal [Lead (Pb), Zinc (Zn), Cadmium (Cd), Iron (Fe), Copper (Cu), Chromium (Cr) and Cobalt (Co)] contamination using atomic absorption spectroscopy. The levels of Pb, Cd and Cr in paper wastewater were all above the safe limit for FAO standards for wastewater quality for irrigation. The concentration of Pb in Swiss chard and Green peeper was exceeded the permissible limits. The study reveals that Pb metal contamination in the study area which poses health risk with time unless an urgent step is taken by relevant agencies to address this issue.