Agroprocessing constitutes sizeable industries in the Eastern Africa region discharging wastes into the environment. Proper management of industrial waste is perceived as expensive and enforcement of laws is weak. Generally, there is low awareness of environmental and socio-economic consequences of polluting the environment. The Banana Investment Limited (BIL) in Arusha Tanzania which produces banana wine from ripe bananas was discharging untreated wastewater into the environment. This project aimed at treating the BIL wastewater to meet environmental standards and recover nutrients, water and energy. The feeding wastewater to the up-flow anaerobic sludge blanket (UASB) with flow rate of 62.4 m3/d had concentrations (mg/L) of chemical oxygen demand (COD) (4,959.3 ± 388.7), BOD5 (1,453.7 ± 110.3), total suspended solids (TSS) (2,431.0 ± 190.5), NH4+ (7.2 ± 1.1), NO3 (23.4 ± 3.2), PO43− (5.12 ± 0.73), volatile fatty acid (0.60 ± 0.09), and Alkalinity (60.00 ± 8.98). After 17 months of operation the system achieved removal efficiencies (%) of COD (99.0), BOD5 (98.6), TSS (96.0), NO3 (88.7), PO43− (50.8). There was a net generation of NH4+ (387.8%) in the system. The biogas produced in the UASB is collected at a rate of 163 m3/d and is used in the boiler at BIL. The dried sludge and the treated water are used for irrigation. The study concluded that integrating the bio-digestion process with polishing stage for water, nutrient and energy recovery ensures compliance to environmental law and provides incentive to treat wastewater while also mitigating greenhouse gases.

INTRODUCTION

The agrobased industry in Tanzania is reported to be growing (Bakili et al. 2014) due to increased emphasis on value addition to agricultural production, the main stay of Tanzania's economy. The main agrobased industry in Tanzania falls into the categories of sugar processing, brewing, sisal processing, slaughterhouses, winery, tanning, fruit and vegetables, non alcoholic beverages etc. The agro industries are characterised by effluents that contain high levels of organics and nutrients. These large volume high strength organic effluents if discharged into the environment untreated or partially treated, cause oxygen depletion in water bodies, bad odors, emit methane which is a greenhouse gas (GHG) into the atmosphere. On the other hand, if these effluents are properly managed they can serve as a source of energy (Rajeshwari et al. 2000), nutrients and recycled water for other applications such as in agriculture (Qadir et al. 2010). Capturing of the methane gas through biogas not only provides a source of energy but also mitigates the effects of the greenhouse since it is combusted into CO2 when biogas is used to generate energy. Biogas is a methane-rich fuel gas produced by anaerobic breakdown or digestion with the help of methanogenic bacteria under oxygen free environment (Dhadse et al. 2012). Anaerobic digestion of organic matter for biogas production has been widely used in the treatment of wastewater (Vadlani & Ramachandran 2008) and can be used as depollution tool as well as for energy recovery.

The Banana Investment Limited (BIL) is one of the agroprocessing industries in Tanzania, producing banana wine and other alcoholic beverages mainly from ripe bananas. The factory faced a challenge of discharging untreated wastewater into the environment. Winery effluents are known to be highly loaded and can produce severe negative effects to the environment if they are discharged without sufficient treatment (Molina et al. 2007). However, these wastes are good sources of biogas due to the presence of highly degradable organic matter (Saleh & Mahmood 2004). BIL uses 3,600 m3 of firewood, and 252 m3 of industrial diesel oil (IDO) each year for its banana pulp boiling processes and steam generation. Since banana winery effluent (BWE) produced from ripe bananas was hardly reported in academic literature, Bakili et al. (2014) reported the composition of raw BWE (Table 1).

Table 1

Composition of Raw BWE collected from BIL

Parameter Units Concentration 
pH  6.9 ± 0.06 
Chemical Oxygen Demand g/L 5.3 ± 0.1 
Total Organic Carbon g/L (34.6 ± 2.18) × 10−3 
Total Kjeldahl Nitrogen g/L (7.7 ± 1.02) × 10−3 
Total Suspended Solids g/L 2.2 ± 0.173 
Phosphate (PO43−g/L (7.3 ± 1.0) × 10−3 
Carbon to Nitrogen (C/N) ratio  5:1 
Parameter Units Concentration 
pH  6.9 ± 0.06 
Chemical Oxygen Demand g/L 5.3 ± 0.1 
Total Organic Carbon g/L (34.6 ± 2.18) × 10−3 
Total Kjeldahl Nitrogen g/L (7.7 ± 1.02) × 10−3 
Total Suspended Solids g/L 2.2 ± 0.173 
Phosphate (PO43−g/L (7.3 ± 1.0) × 10−3 
Carbon to Nitrogen (C/N) ratio  5:1 

Bakili et al. (2014) concluded from batch laboratory experiments that BWE does not need an addition of nitrogen-rich substrate to improve biogas production process but requires addition of carbon-rich substrate to improve its C: N ratio and thus increase biogas production efficiency.

There are confounding factors that enhance theprocesses in up-flow anaerobic sludge blanket (UASB) which are the strength and composition of industrial wastewater, temperature and diurnal fluctuations (Chernicharo 2006; Foresti et al. 2006). The degradation of organic matter in UASB as anaerobic reactor involves hydrolysis, acidogenesis, acetogenesis and methanogenesis. This is carried out by consortia of anaerobic microorganisms whereby during hydrolysis, complex organic matter is broken into carbohydrates, lipids and proteins.

These are further degraded into amino acids, sugars, carboxylic acids and alcohols during acidogenesis. Higher organic acids including acetate, butyrate and lactate, form during acidogenesis while methane and carbon dioxide are formed during methanogenesis (Esposito et al. 2012).

The BIL established a full scale integrated process involving UASB and constructed wetland to treat its wastewater to meet compliance levels and generate biogas for internal use. The treated effluent is directed for irrigation while the sludge is dried and applied on land to provide residue nutrients and organic carbon. The system started operation in 2014 and in this paper the performance of the full scale integrated upflow anaerobic sludge blanket-constructed wetland (UASB-CW) system after operating for 17 months is presented.

MATERIALS AND METHODS

Site description

The BIL is located at old Moshi road, Kijenge village in Arusha City which lies below the equator between latitudes 2° and 6° and Longitudes between 35° and 38° east of Greenwich. The BIL is found in between Olorien and Olkeiyan wards and Kijenge stream flows within the BIL premises.

The effluent process flow diagram

The integrated UASB-CW system at BIL is composed of four major stages with specific functions. The pretreatment stage which comprises of bar screen with dimensions: length of 3 m, width of 1 m, depth of 1 m and three bar screens placed at 1 m each with space of 20 mm, 10 mm and 5 mm respectively; transfer sump of length and width of 3.5 m and depth of 3.5 m; equalization tank with two compartments of length and width of 6 m and height of 3 m each and primary clarifier of diameter 4 m and height of 2.5 m. Bar screening is for the removal of coarse solid materials, transfer sump for collection raw wastewater and feed to the equalization tank by a pump, the primary clarifier enhances settling of the fine suspended solids. The second stage is the bio-digester which is an UASB reactor with diameter of 10 m, height of 8 m and 0.5 m height of hoods for biogas collection used for organic matter degradation and biogasification. The gas holder has diameter of 3 m and height of 2.5 m while the biogas collector has diameter of 2.5 m and depth of 2.5 m. The constructed wetland is baffled with two cells in series each with length of 30 m, width of 7.5 m and depth of 1 m. This unit is used for tertiary treatment stage for polishing the wastewater to meet environmental requirements. The CW is packed with clay soil as bed liner, then packed with basalt gravel sized ½–3/4 inches (12.5 mm–20 mm) and planted with papyrus type of vegetation to mimic the natural wetland systems. CW was chosen for polishing due to its good efficiency in further removal of organic matter, removal of nitrogen species and further removal of suspended matter to meet the allowable standards for discharge aimed for irrigation (Njau et al. 2003). The fourth stage is the sludge drying bed used for processing the sludge for agricultural application as organic fertilizer. The BIL effluent treatment process flow is shown in Figure 1.
Figure 1

Effluent treatment process flow diagram.

Figure 1

Effluent treatment process flow diagram.

Sampling

The integrated UASB-CW at BIL was monitored on daily basis for the environmental parameters. The average monthly samples were taken since January, 2015 up to May, 2016 that constitute a range of samples taken and analysed to monitor the performance of the systems.

The sampling schedule was as following: sample-1 was effluent from primary clarifierfed into UASB, sample-2 was effluent from UASB fed into CW, and sample-3 was effluent from CW released for irrigation and fishpond.

Laboratory analysis

The parameters that are indicators of environmental pollution in water receiving bodies namely biochemical oxygen demand (BOD5), chemical oxygen demand (COD), total suspended solids (TSS), Nutrients (Nitrate, NO3, and Ortho Phosphate, PO43−), concentrations, temperature, and pH were monitored. The parameters mentioned above were analyzed using analytical instruments and procedures as per methods described in standard methods (APHA 2012) for examinations of water and wastewater. Process parameters namely volatile fatty acids (VFA), temperature, pH and alkalinity were also monitored in the UASB.

The analytical instrument, COD Multiparameter Bench Photometer was used to measure parameters with respective to particular wavelengths after adding the reagents appropriate for each parameter such as COD at 610 nm, Nutrients (Nitrate, NO3, and Ortho Phosphate, PO43−) at 525 nm according to HANNA instruction Manual (Model HI 83099 HANNA). BOD5 was analyzed using Oxitop IS 12 BOD5 incubator. Further, pH and temperature was measured using the combined pH and temperature meter (Model HI 9024 HANNA). TSS, was analyzed using drying oven method for operation at 103–105 °C.

RESULTS AND DISCUSSION

The measured average banana winery wastewater produced during the study period is 62.4 m3/d. This is a fraction of the design value of the integrated UASB-CW system which was 200 m3/d. This huge difference is attributed partly to improvement of bottle washing processes and banana wine filling into bottles. The replacement of manual bottle washer and banana wine filling facilities by automated machines reduces generation of 137.6 m3/d volume of wastewater caused by uncontrolled washing of bottles and cleaning of floor from spillages of banana wine.

Table 2 shows the operation conditions of UASB. The UASB at BIL was designed to operate in the mesophilic range of temperatures but it was observed during monitoring that in average it was operating at lower temperatures around 29.3 ± 1.7 °C. However, laboratory batch data obtained by Bakili et al. (2014) showed that the optimum temperature is 35 °C.

Table 2

Operation conditions of the UASB

Parameter Maximum Minimum Average 
Temperature, T (oC) 32.3 26.5 29.3 ± 1.7 
pH 7.34 7.00 7.26 ± 0.09 
Parameter Maximum Minimum Average 
Temperature, T (oC) 32.3 26.5 29.3 ± 1.7 
pH 7.34 7.00 7.26 ± 0.09 

N = 17.

Table 3 shows the performance of the UASB in the removal of the key parameters. The UASB was excellent in the removal of COD (97.5%), BOD5 (97.5%), TSS (95.3%), NO3 (98.2%). Alkalinity was reduced by mere 32.6%. PO43− was practically unchanged in the UASB. This is understandable as the main pathways for the removal of PO43− is through uptake by plants and living organisms, attachment to substrate materials etc. However, Schuler & Jenkins (2003) reported two enhanced biological phosphorus removal pathways from wastewater which are glycogen-accumulating metabolism (GAM) and polyphosphate-accumulating metabolism (PAM). The (GAM)-microorganisms dominate in anaerobic reactor treating low influent phosphorus/COD ratio while (PAM)-microorganisms dominate in anaerobic reactors fed by high influent phosphorus/COD ratio (Schuler & Jenkins 2003). Nevertheless, the small increase in the concentrations of ortho-phosphates (PO43−) in the UASB effluent might be due to domination of GAM microorganismsin the UASB that slowly hydrolysed organic phosphorus causing small increment of the phosphates in liquid phase (Schuler & Jenkins 2003). Further study done by Seviour et al. (2003) reported that during the anaerobic phase, polyphosphate accumulating organisms preferentially assimilate VFAs, such as acetate which is used to synthesize intracellular energy reserves poly-β-hydroxyalkanoate (PHA) in the process using intracellular poly (P) as energy source, and releasing phosphate into the bulk wastewater.

Table 3

Performance of the UASB

Parameter In Out UASB Efficiency (%) 
COD (mg/L) 4,959.3 ± 388.7 125.9 ± 17.3 97.5 
BOD5 (mg/L) 1,453.7 ± 110.3 36.4 ± 4.6 97.5 
TSS (mg/L) 2,431.0 ± 190.5 114.0 ± 6.1 95.3 
NH4+ (mg/L) 7.2 ± 1.1 66.5 ± 13.9 −823.2 
NO3 (mg/L) 23.4 ± 3.2 0.41 ± 0.06 98.2 
PO43− (mg/L) 5.12 ± 0.73 5.14 ± 1.06 −0.4 
VFA (mg/L) 0.60 ± 0.09 3.81 ± 1.29 −534.4 
Alkalinity (mg/L) 60.00 ± 8.98 40.46 ± 6.74 32.6 
Parameter In Out UASB Efficiency (%) 
COD (mg/L) 4,959.3 ± 388.7 125.9 ± 17.3 97.5 
BOD5 (mg/L) 1,453.7 ± 110.3 36.4 ± 4.6 97.5 
TSS (mg/L) 2,431.0 ± 190.5 114.0 ± 6.1 95.3 
NH4+ (mg/L) 7.2 ± 1.1 66.5 ± 13.9 −823.2 
NO3 (mg/L) 23.4 ± 3.2 0.41 ± 0.06 98.2 
PO43− (mg/L) 5.12 ± 0.73 5.14 ± 1.06 −0.4 
VFA (mg/L) 0.60 ± 0.09 3.81 ± 1.29 −534.4 
Alkalinity (mg/L) 60.00 ± 8.98 40.46 ± 6.74 32.6 

N = 17.

In the UASB however, there was an increase of NH4+ by 823.2% and VFA by 534.4%. Ammonia is known to be generated in anaerobic environments similar to those found in UASB through a disimilatory NO3 reduction. Several microbial processes compete for nitrate, denitrification, dissimilatory nitrate reduction to ammonium and anaerobic ammonium oxidation (Kraft et al. 2011). When there is high levels of S2− under anaerobic conditions, sulphide stimulates DNRA, by serving as an electron donor, and depresses denitrification, by repressing NO and N2O reductase (Rutting et al. 2011). Similarly anaerobic fermentation of organic nitrogen leads to ammonia formation due to the presence of sufficient organic carbon by supporting respiration or fermentation, that regulates the population of DNRA bacteria under anaerobic conditions (Rutting et al. 2011).

The VFA was increased from 0.60 ± 0.09 mg/l to 3.81 ± 1.29 mg/l (534.4%) which signifies that the microorganisms were able to utilize the intermediate organic acids like acetate, butyrate and lactate during acidogenesis for biogas formation at methanogenesis stage. The increase in VFA in anaerobic conditions, is reported by Ravindranath et al. (2010), to be the result of the acidogenic bacteria converting the organic matter to VFA. However, the VFA to alkalinity ratio for BIL UASB was observed to be 0.1. Jayantha & Ramanujam (1996) however, reported that for healthy anaerobic reactor, the ratio of VFA to alkalinity should be 0.3 to 0.4 so that the available alkalinity is sufficient to counteract VFA and maintain near neutral pH.

Table 4 shows the performance of the BIL constructed wetland. The CW was good in further removal of COD (62.0%), BOD5 (45%), TSS (76.7%), NH4+ (43.5%) and PO43− (60.4%). Nitrate however was increased in the system by 585.3%. Increase of nitrate in the CW can only be attributed to nitrification of ammonia. Dissolved ammonia in presence of O2 is known to be oxidised to nitrite and nitrate by bacteria Nitrosomonas and Nitrobacter respectively in a two step process that yield energy. 
formula
Constructed wetlands are known to posess anoxic and oxic zones (D'Angelo 2002). Two things are clear from these results. One is the importance of CW in the integrated process. It is a fact from these results that UASB alone would release high levels of ammonia and no reduction of P. The second is the fact that the CW does not offer complete nitrification of ammonia to nitrate. This is probably due to the limitation of dissolved oxygen in the sub-surface flow wetland despite the pumping of oxygen through the root system by wetland plants. This problem can easily be solved by introducing an aeration step between CW and UASB so as to increase the level of oxygen in the system.
Table 4

Performance of the CW

Parameter In Out CW Efficiency 
COD (mg/L) 125.9 ± 17.3 47.8 ± 5.6 62.0 
BOD5 (mg/L) 36.36 ± 4.09 20.00 ± 4.38 45.0 
TSS (mg/L) 114.05 ± 6.02 26.56 ± 2.53 76.7 
NH4+ (mg/L) 62.16 ± 8.56 35.14 ± 4.87 43.5 
NO3 (mg/L) 0.38 ± 0.05 2.64 ± 0.37 −585.3 
PO43− (mg/L) 6.35 ± 0.95 2.52 ± 0.37 60.4 
Parameter In Out CW Efficiency 
COD (mg/L) 125.9 ± 17.3 47.8 ± 5.6 62.0 
BOD5 (mg/L) 36.36 ± 4.09 20.00 ± 4.38 45.0 
TSS (mg/L) 114.05 ± 6.02 26.56 ± 2.53 76.7 
NH4+ (mg/L) 62.16 ± 8.56 35.14 ± 4.87 43.5 
NO3 (mg/L) 0.38 ± 0.05 2.64 ± 0.37 −585.3 
PO43− (mg/L) 6.35 ± 0.95 2.52 ± 0.37 60.4 

N = 17.

Table 5 provides the overall performance of UASB –CW integrated system. It is clear from Table 5 that the integrated UASB-CW system is excellent in the overall removal efficiencies of COD (99.0%), BOD5 (98.6%), TSS (96.0%) NO3 (88.7%) and PO43− (50.8%). The PO43− reduction in constructed wetland is due to chemo-sorption processes that bind it with clays and the substrate materials of the wetland through ion exchange process with iron and aluminium oxides to form new mineral compounds (Fe- and Al-phosphates), which are potentially very stable, affording long-term storage of phosphorus (Kadlec & Knight 1996). Ammonia however, was generated in the system (387.8%). It is obvious that in order to ensure adequate ammonia removal and aeration stage between the UASB and CW is required. It is also important to note that the effluent has met limits set by the Tanzanian Bureau of Standards (TBS) for discharge into receiving waters.

Table 5

Overall Performance of the integrated UASB-CW

Parameter In Out Allowed limits (TBS) Overall Efficiency (%) 
COD (mg/L) 4,959.3 ± 388.7 47.8 ± 5.6 60 99.0 
BOD5 (mg/L) 1,453.7 ± 110.3 20.00 ± 4.38 30 98.6 
TSS (mg/L) 2,431.0 ± 190.5 26.56 ± 2.53 100 96.0 
NH4+ (mg/L) 7.2 ± 1.1 38.1 ± 4.87 N.I. 4.29 
NO3 (mg/L) 23.4 ± 3.2 2.64 ± 0.37 50 88.7 
PO43− (mg/L) 5.12 ± 0.73 2.52 ± 0.37 6.0 50.8 
Parameter In Out Allowed limits (TBS) Overall Efficiency (%) 
COD (mg/L) 4,959.3 ± 388.7 47.8 ± 5.6 60 99.0 
BOD5 (mg/L) 1,453.7 ± 110.3 20.00 ± 4.38 30 98.6 
TSS (mg/L) 2,431.0 ± 190.5 26.56 ± 2.53 100 96.0 
NH4+ (mg/L) 7.2 ± 1.1 38.1 ± 4.87 N.I. 4.29 
NO3 (mg/L) 23.4 ± 3.2 2.64 ± 0.37 50 88.7 
PO43− (mg/L) 5.12 ± 0.73 2.52 ± 0.37 6.0 50.8 

N = 17; N.I. Not Indicated in the Standards.

Biogas produced

The volume of biogas produced was in average of 163 m3 in a day which is used in boiler for steam generation. Such amount of biogas produced replaces 116.4 L/d of IDO used in boiler for steam generation. This makes BIL to save from buying 3,492.86 litres of IDO per month equivalent of 4, 890,000 Tanzanian shillings per month (2,173.33 USD/month).

Sludge quality

The sludge from the equalization tank, the primary clarifier and UASB was dried in a sludge drying bed for reuse as organic fertilizer. The results in Table 6 indicate the composition of the dried sludge that the BIL winery sludge has good levels of N, P, K, and organic carbon. The near neutral pH indicates that the sludge shall not affect the soil pH when applied on land. Khalid et al. (2012) reports the composition of organic matter and major nutrients in sewage sludge where the values reported from Pakistan were: Organic Matter (19,400 mg/kg), available N (5,200 mg/kg), available P (70 mg/kg) and available K (288 mg/kg). Comparing BIL winery sludge with these values it is clear that the latter is richer than sewage sludge in Organic Matter (69,500 mg/kg), available N (7,200 mg/kg), and available P (169.44 mg/kg). However, the results show that the BIL winery sludge is poorer in available K (9.98 mg/kg).

Table 6

Composition of BIL banana winery sludge

S/No. Parameter Units Value 
1. pH – 7.8 
2. Colour – Blackish 
3. Organic Carbon, OC % or (g/kg) 6.95 or (69.5) 
4. Nitrogen, N % or (g/kg) 0.72 or (7.2) 
5. Phosphorus, P mg/kg 169.44 
6. Potasium, K mg/kg 9.98 
7. Calcium, Ca mg/kg 50.45 
8. Iron, Fe g/kg 54.22 
9. Manganese, Mn mg/kg 220 
10. Magnesium, Mg mg/kg 200 
11. Copper, Cu mg/kg 9.97 
12. Zinc, Zn mg/kg 5.60 
S/No. Parameter Units Value 
1. pH – 7.8 
2. Colour – Blackish 
3. Organic Carbon, OC % or (g/kg) 6.95 or (69.5) 
4. Nitrogen, N % or (g/kg) 0.72 or (7.2) 
5. Phosphorus, P mg/kg 169.44 
6. Potasium, K mg/kg 9.98 
7. Calcium, Ca mg/kg 50.45 
8. Iron, Fe g/kg 54.22 
9. Manganese, Mn mg/kg 220 
10. Magnesium, Mg mg/kg 200 
11. Copper, Cu mg/kg 9.97 
12. Zinc, Zn mg/kg 5.60 

There are many advantages of applying sludge on land due to the fact that it increases beneficial soil organisms, reduces plant pathogen (Abawi & Widmer 2000), improves water holding capacity (Wells et al. 2000) and provides array of nutrients to soil (Tejada et al. 2001). Moreover, compost and sewage sludge has a tendency of maintaining higher soil organic matter levels that eventually help in carbon sequestering to mitigate GHG emissions (Khalid et al. 2012). The BIL winery sludge as such has valuable input in a view of its high organic matter content and rich macro and micro nutrients essential for maintaining soil fertility and productivity.

CONCLUSION

Results of this study have clearly demonstrated that for agro based wastewater, integrating UASB with constructed wetland can produce efficient treatment to meet environmental standards for water discharge into the environment. Combination of water, nutrients and energy recovery promotes compliance on the part of the polluter by offering incentive to treat the wastewater through the energy recovery route while also mitigating GHGs. It also creates useful by-products such as the sludge which is a source of nutrients and organic carbon and water. These by-products can be used within the agro based industry if they have farms in the neighbourhood or they can build good public relations with the surrounding communities who need to supplement artificial fertilisers for their subsistence agricultural activities. It is clear from the results however, that an aeration step is required between the UASB and the constructed wetland to convert NH4+ to NO3 before entering the constructed wetland.

ACKNOWLEDGEMENTS

This work was accomplished with the generous funding support of Sida through the Bio-Innovate project. The Banana Investment Limited (BIL) also supported this project financially.

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199
202
.