19-10-2012, 03:53 PM
The use of Moringa oleifera seed as a natural coagulant for water and wastewater treatment
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Abstract
Moringa oleifera Lam. (M.oleifera) is a multipurpose tree of considerable potential and its cultivation is currently being actively promoted in many developing countries. The various uses and products of the tree are given.
Seeds of this pan tropical tree contain water soluble, positively charged proteins that act as an effective coagulant for water and wastewater treatment (molecular weight 13 kDa and isoelectric pH 10-11).
The Environmental Engineering Group at Leicester University, UK have evaluated the potential of this plant derived material for a range of treatment applications in the laboratory and in the field using water soluble extracts of the crushed kernel and of presscake (solids residue remaining after oil extraction – the oil content of the seed is high as is the quality). Significant performance test results are presented.
M.oleifera seed was applied to a pilot scale treatment system (gravel bed flocculation – plain sedimentation – sand filtration) as a primary coagulant to clarify an African river water in the wet season. The source water exhibited a relatively constant turbidity of 400 NTU over the test period. Over 90% turbidity removal was achieved in the sedimentation stage at a dosage of 100 mgL-1. The M.oleifera dose is expressed as equivalent seed kernel material. The filtrate turbidity was maintained well below 5 NTU. Subsequent trials were conducted on the same river water in an adjacent municipal works comprising upflow contact clarifiers and rapid gravity filters operating at 60 m3h-1. Treatment performance was comparable to that of using aluminium sulphate as coagulant. Inlet turbidities in the range 270-380 NTU were consistently reduced to below 3 NTU.
A comprehensive study was undertaken with the same source water in the dry season to evaluate the potential of using M.oleifera coagulant within a contact flocculation-filtration (CFF) pilot plant rig. For the purposes of this paper, CFF is defined as the high rate filtration process for relatively low turbidity raw waters wherein the coagulant is dosed immediately prior to entry to the sand bed – essentially single stage treatment. Flocculation and subsequent deposition occurs entirely within the filter bed. A wide range of operating conditions was evaluated in order to establish the useful ‘working envelope’ for this process combination (filtration rates 5-20 mh-1, initial turbidities 20-50 NTU). The main results of this field study are summarised.
As a coagulant within chemically enhanced primary sedimentation (CEPS) of a mixed domestic/industrial wastewater, M.oleifera dosed at 150 mgL-1 gave additional removals (compared to a plain sedimentation control) of 40% for biochemical oxygen demand (BOD) and chemical oxygen demand (COD) and in excess of 80% for suspended solids (SS). Subsequent laboratory work at the University of Ghent coupled an upflow anaerobic sludge blanket reactor (UASB) to CEPS. M.oleifera coagulant in the CEPS pre-treatment unit beneficially increased the ratio of soluble COD to volatile SS by a factor of 10 compared to plain sedimentation and 3 when dosing ferric chloride as coagulant. The UASB yielded more biogas and gave 71% removal of total COD at 2 hours hydraulic residence time. This compared with 54% removal of total COD at the same residence time when ferric chloride was used. Brief details of a further study at Ghent are given relating to the enhanced start up of a UASB reactor with the addition of a water extract of M.oleifera seed.
Introduction
The M.oleifera tree is a native of Northern India, which now grows widely throughout the tropics. English vernacular names include drumstick (shape of the pods) and horseradish (taste of the roots). It may be propagated from seeds or cuttings, grows well even in poor soils requiring minimal horticultural attention and is able to survive long periods of drought. It grows rapidly -growth of up to 4 metres in height, flowering and fruiting were all observed within one year during trials near Nsanje in Southern Malawi Extended and multiple harvests in a single year are evident in many parts of the world. The many products and numerous uses of the tree are given in Table 1.
Moringa derived coagulants - general
River water drawn for human consumption and general household use can be highly turbid particularly in the rainy season. River silt is churned into suspension and run off from fields and other surfaces carries solid material, bacteria and other microorganisms into the river. It is of paramount importance to remove as much of this suspended matter as possible prior to a disinfection stage and subsequent consumption. This can generally only be achieved by the addition of coagulants to the raw water, within a controlled treatment sequence. In many developing countries, proprietary chemical coagulants, such as aluminium sulphate and synthetic polyelectrolytes are either not available locally or are imported using foreign exchange.
A viable alternative is the use of crushed seed of M.oleifera as a natural coagulant. The seed pods are allowed to dry naturally on the tree prior to harvesting. The mature seeds are readily removed from the pods, easily shelled and then may be crushed and sieved using traditional techniques such as those employed for the production of maize flour The crushed seed powder, when mixed with water, yields water soluble proteins that possess a net positive charge (molecular weight 13 kDa and isoelectric
pH 10-11). Dosing solutions are generally prepared as 1-3% solutions and are filtered prior to application to the untreated water (Sutherland et al, 1990).
River water treatment at pilot scale
It is now regarded as axiomatic that both water and wastewater technology for developing countries must be no more complex than strictly necessary and be robust and inexpensive to install and maintain. A prototype treatment works was designed founded on this philosophy. The pilot plant was constructed within the grounds of the Thyolo Water Treatment Works, the works being controlled by the Ministry of Works and Supplies Water Department of the Malawi Government. The pilot plant is shown schematically in Figure 1. River water is pumped at 1 m3h-1 from the River Nswadzi to a header tank where the M.oleifera seed solution is introduced into the turbulent jet of incoming water and mixed hydraulically. An 18-minute flocculation period is provided within the upflow gravel bed flocculators prior to plain sedimentation. A rapid gravity filter removes any residual floc carried over from the sedimentation tank. (All the units were locally fabricated in sheet steel of gauge SWG 16).
The system was successfully commissioned during the rainy season with the source river exhibiting turbidity levels in excess of 400 NTU throughout the study period. Figure 2 shows the pilot plant performance for one test conducted over a 7-hour period. At a dose of 100 mgL-1, raw water turbidity of 400 NTU was reduced to approximately 30 NTU in the outflow from the sedimentation tank. As the sand filter ’worked in’, the final turbidity was approaching 1 NTU when a regional power failure terminated the test.
Figure 3 is a composite of all the tests performed over the study period. Solids removal within the plant was consistently above 90% following the gravel bed flocculation stage and plain horizontal flow sedimentation. Subsequent rapid gravity sand filtration gave final, treated water turbidity generally well below 5 NTU. M.oleifera seed dose ranged from 75-250 mgL-1 depending on the initial raw water turbidity (Folkard et al,1993).
River water treatment at full scale
During the following wet season the main ThyoIo works was operated using M.oleifera solution as coagulant. The works comprise upflow contact clarifiers followed by rapid gravity filters and chlorination. The clarifiers were in a state of some disrepair with the impeller drives and chemical feed pumps inoperative. Under normal operation, alum solution is introduced into the incoming flow of 60 m3h-1 by simple gravity feed. Comparable treatment performance with alum was achieved. This was the first time that M.oleifera had been successfully used as a primary coagulant at such a scale with the treated water entering supply (Sutherland et al, 1994). Figure 4 shows the data for a 7.5-hour test with the main works flow at 60 m3h-1 and the seed dose at 75 mgL-1. The inlet turbidity is over 325 NTU and the output from the filter consistently below 2 NTU
M.oleifera seed for the full-scale trials was purchased from enthusiastic villagers in the Nsanje region. This was viewed as a temporary yet very welcome new source of cash income in what is a poor rural community of Southern Malawi. The tree is widely cultivated in this area, being highly prized as a source of fresh, green vegetable.
Treatment of eutrophic water
Treatment studies were also conducted at bench scale using a eutrophic lake water serving the main treatment works to Harare, Zimbabwe (Sutherland et al, 1995). The impounded water contained much light organic matter in suspension due to high algal growth and exhibited relatively low turbidity throughout the year. As such, the water is problematic to treat consuming significant quantities of alum (as primary coagulant) and activated silica (as weighting agent). Alum floc carry over from the clarifiers causes "filter blinding" and the sludge from the clarifiers is voluminous, difficult to dewater and presents pollution problems on discharge to the receiving water M.oleifera in combination with sodium bentonite as weighting agent produced a final water quality equivalent to that produced using the conventional chemical coagulants The sludge was significantly more compact and represents a potentially useful output as a soil conditioner/fertiliser.
M.oleifera coagulant for wastewater treatment applications
As a coagulant within chemically enhanced primary sedimentation (CEPS) of a mixed domestic/industrial wastewater, M.oleifera dosed at 150 mg l-1 gave additional removals (compared to a plain sedimentation control) of 40% for biochemical oxygen demand (BOD) and chemical oxygen demand (COD) and in excess of 80% for suspended solids (SS) (Folkard et al, 1999)
Subsequent laboratory work at the University of Ghent coupled an upflow anaerobic sludge blanket reactor (UASB) to CEPS (Kalogo et al, 2000). The UASB process relies on the propensity of anaerobic biomass to aggregate into dense flocs or granules over time Mixing is achieved by pumping influent wastewater from an entry at the base upwards through the sludge blanket. Figure 9 is a schematic representation of a UASB. Above the blanket, finer particles flocculate and in the upper settlement zones they settle back as sludge in the blanket. Thus, washout of biomass is prevented. The biogas, which has poor solubility in water, is separated at the top of the reactor.
Domestic wastewater treatment in UASB reactors has proved particularly effective in tropical regions of the world. Effective removal of organic matter and suspended solids is evident at reduced excess sludge volume compared to aerobic treatment. The system is compact, requires minimal energy inputs and does not require support media normally associated with anaerobic systems (de Sousa et al, 1996)