Water is a precious natural resource vital for sustaining life. It is in a continuous circulation movement (i.e., hydrological cycle), and is not uniformly distributed in time and space. Due to its multiple benefits and the problems created by its excesses, shortages and quality deterioration, water, as finite resource requires special attention (Pinderhughes, 2004).
Water treatment usually comprises water clarification and disinfection processes (Suarez, 2003). During water treatment, the combination of coagulation, flocculation, sedimentation, filtration and disinfection processes are used to facilitate water treatment.
This combination, is needed, so as to improve the quality of raw water depending on the type of water quality problems present, the desired quality of the treated water, the costs of different treatments and the size of the water system (Kalibbala, 2007)
Water treatment using biological materials can be effective in providing clean water at very minimal price which can be affordable by each households. Biological materials that are locally available, example Moringa Oleifera plant has been used in many areas of developing countries, as a means for treating of water.
Coagulation process depict the agglomeration of small precipitate particles into larger precipitate particles, known as floccs. Coagulants neutralizes net electrical repulsive force between precipitate particles and hence facilitate them to settle down. Addition of coagulants to acidic drainage waters enhance increased number, hence flocs density increases, which results to increase of inter particle contact due to Brownian motion. This kind of motion promote agglomeration of colloidal particles to give large flocs (Qasim, 2000)
Flocculation is due to combination of small particles by bridging the particles with chemicals (Skousen, 1996).
Essentially, coagulants facilitates the formation of metal precipitate flocs, and flocculants enhance floc by making it heavier and more stable. For this reason, flocculants are sometimes referred to as coagulant aids at water treatment operations (Tillman, 1996)
2.3 chemical coagulants
2.3.1 Iron salts
Iron coagulants are ferric sulphate (Fe2 (SO4)3), ferrous sulphate (FeSO4) and ferric chloride (FeCl2). Iron compounds are generally cheaper, produce a heavier floc, and perform over a wider pH range than aluminium coagulants (Tillman, 1996). However, iron coagulants are not used as much as aluminium due to staining equipment, corrosiveness, and they require more alkalinity than alum. Ferric sulphate is active over a wider pH range (4.0-6.0, 8.8-9.2) than ferrous sulphate (8.8-9.2) and produces heavier flocs which settle more quickly.
2.3.2 Aluminium salts
Aluminium coagulants which are often used includes, aluminium sulphate (alum), sodium aluminates, and polyaluminium chloride. Alums are available in various grades, with a minimum aluminium content expressed as 17 % of A12O3. Liquid alum is about 49 % solution, or approximately 8.3 % by weight aluminium as A12O3. Alum coagulation works best for a pH range of 5.5 to 8.0.
This is usually not considered as an effective coagulant because it does not produce flocs like salts of iron and aluminium. It reacts with phosphorous and bicarbonate compounds in water to adjust pH causing precipitation of calcium carbonate and magnesium hydroxides (Cosidine, 1974).
2.3.4 Activated silica
The nature of interaction with suspended solids is somehow analoguos to that of polyelectrolytes but differs by lacking the long flexible chains and is therefore denser. They are usually referred to as weighting agents that promote settling of flocs. Dosages are about 20-60 % of alum dose used for coagulation. They have been used with or without alum to achieve clarification in lime water-softening plants (Cosidine, 1974).
Polyelectrolytes are water-soluble organic polymers consisting of repeating units of smaller molecular weights chemically combined to form larger molecules of colloidal size each carrying electrical charges or ionized groups. They can be either natural or synthetic and can be used as both primary coagulants and coagulant aids (Hashimoto et al., 1991). Polyelectrolyte primary coagulants are cationic with high charge density and low molecular weight, while synthetic polyelectrolyte coagulant aids have relatively high molecular weights and facilitate flocculation through inter-particle bridging (Gregory and Duan, 2001).
2.4 Alum as a chemical coagulant
Alum (Al2 (SO4)2.14H2O) is available commercially in industrialized countries in lumps, ground or liquid form. It is a basic product of the reaction between sulphuric acid and a mineral despite such as bauxite. Lump or ground alum whether purified or not contain not less than 9.0 % of available water-soluble aluminium as Al or 17 % as Al2O3 (AWWA, 1990).
Chemical coagulation with alum like any other form of coagulant is aimed at achieving the following objectives:
Removal of turbidity, inorganic or organic
Removal of harmful bacteria and other pathogens
Removal of colour, taste and odour producing substances.
Alum is a relatively inexpensive coagulant if local production is possible. In most developing countries, it is imported at substantially increased cost. Treatment plants in these countries must be designed so that alum consumption may be minimised. The dosage of alum may be reduced in some instances by
1. Direct filtration of low turbidity waters
2. Pre-treating excessively turbid river waters
3. Use of coagulant aids
4. Optimum pH adjustment
2.5 Factors affecting coagulation/flocculation
Coagulation and flocculation processes are dependent on numerous inter-related factors, Such factors include the characteristics of the water source, raw water pH, alkalinity and temperature, the type of coagulant and coagulant aids and their order of addition, dose rates of coagulants, the degree and time of mixing provided for chemical dispersion and flocs formation. For water with low alkalinity coagulant can consume virtually all of the available alkalinity, hence lowering the pH to a level that hinders effective treatment, while high alkaline waters may require additional chemicals to lower the pH to values favourable for coagulation (Rossi and Ward, 1993; Kalibbala, 2007).
The performance of the hydrolysing metal salts is significantly influenced by the pH of the solution and they have a good coagulation effect within a certain pH range of the water. The coagulation process in water treatment can be modified to facilitate the removal of dissolved organic matter which has been reported to occur optimally at pH 5-6 and at maximum rate at pH 4 (Gregory and Duan, 2001).
Low temperature affects the coagulation and flocculation process by altering the coagulant solubility, increasing the water viscosity and retarding the kinetics of hydrolysis reactions and particle flocculation. Poly-aluminium coagulants are more effective in cold water than alum, as they are pre-hydrolysed. To achieve effective coagulation, proper mixing is also necessary to allow active coagulant species to be transferred onto turbid water particles (Gregory et al., 1997).
2.7 Types of natural coagulants
2.7.1 Materials of Soil Origin
It has being observed that mineral substances are used as flocculation aid in modern water treatment. A dose of 10 mg/l of bentonite, for instance, together with 10 mg/l of aluminium sulphate yield significantly better results than a higher dose of aluminium sulphate alone. In rural households in developing countries, however, various naturally occurring materials are traditionally used as coagulants. Examples are fluvial clays from rivers and clarifying rock material from desert regions (Jahn, 1984).
2.7.2 Coagulants of plant origin
Vicia faba (Faba vulgaris) – horse bean
This belongs to the family Papilionaceae and largely cultivated in Sudan. Seeds have been used successfully to purify water in arid regions of Sudan. It is known locally as Ful masri (Jahn, 1986).
This is known in Sudan as helba belonging to the family of Papilionaceae. It also largely cultivated in Sudan.
Moringa oleifera (Horse radish or Drumstick tree)
It is believed to have originated from India but now largely cultivated in Sudan and many other countries. The low molecular weight of M. oleifera cationic proteins (MOCP) extracted from the seeds is very useful and is used in water purification, because of its potent antimicrobial and coagulant property which is attributed to the presence of organic polyelectrolyte, carbonic proteins and lectins (Ijarotimi, et al., 2003).
2.7.3 Protein isolation
Protein isolation method depends on a source properties, either liquid or solid sample. For a solid sample the mechanical homogenization process should be used to homogenize the tissue and lysing cells. Methods for lysing cell includes heat treatment and sonication and treatment with detergent. Appropriate detergents helps to extract membrane protein and nuclear proteins, in the same operation chaotropic reagent like urea and guanidine hydrochloride, are used to increase efficiency of extraction as they break down structure of protein hence it dissolve readily in water.
For liquid samples protein is extracted from cell which its present, where the cell should be isolated by centrifugation. The centrifugation of different media with different densities is much useful to isolate proteins dissolved in a given media.
Concentration and precipitation of dilute protein is necessary as precipitated protein is very stable hence increase shelf life of protein. Traditional techniques of precipitation, such as salting out and heat denaturation have being used to precipitate dissolved protein due to their advantage of being simple to use. Instead of salting out, isoelectric precipitation can also be used by lowering pH, where by precipitate will be formed as a pH reaches at isoelectric point (pI) of 10 or above for Moringa Oleifera protein. Isoelectric precipitation can also be used as fractionation method as each protein has its own isoelectric point (Lee, 2016).
2.7.4 Extraction and mechanism of MO seed coagulant
The coagulant extraction at laboratory scale begins by removing the seeds husk, air dried and grinding the seeds kernel using the clean grinder. Pyrolysis activity is mostly observed for activated carbon as shown in Fig 1. Subsequent to grinding, oil is removed (depend on research aim) and mostly the seed gives high coagulation effect once oil is removed. The next is the re-suspension of the seeds powdered in water at different concentration for optimization purpose (1g/100mL to 5g/100mL or stock of 2.5g/50mL). The stock seeds are blended or agitate at high speed for 2-5 minutes to extract the coagulant agent, then the muslin cloth is normally used to filter the paste and used as a coagulant. (Mohammed Sulaiman, 2017)
Also extraction of the proteins using 1M sodium chloride solution gave enhanced coagulation at significantly reduced dosage compared to water extracted material – 95% turbidity reduction at 4 ml L-1 compared to 78% reduction at 32 ml L-1 for a prepared test water comprising kaolin in water of initial turbidity 50 NTU (the dosage being expressed as volume of 1% stock seed solution, (Okuda, 1999)). The improvement in extraction is attributed to the salting-in mechanism whereby increased ionic strength gives increased protein solubility. The extraction of seed proteins in other salts gave similar improvements.
Figure. 1. Coagulant extraction process (Mohammed Sulaiman, 2017)
2.7.5 Solubility of Moringa Oleifera protein
During clarification the minimum solubility of MO proteins occurred at pH 9, 2, and 3 whereas the maximum protein solubility was observed at pH 7 and 5 (Ahmed, 2016). Similar results were reported by (O.S. Ijarotimi, 2013) on protein concentrates of common leafy vegetables. In another study (Okuda, 1999) have studied the effect of pH on protein solubility of akee pulp and seed flours. The minimum solubility was found to be at pH 3 and 7 for seeds and for pulp at 4 and 10. This indicates that two different isolates might be possible, at pH 4.0 and 10.0, for the pulp extraction, while, for the seeds, one would be extractable at pH 3.0 and another at pH 7.0