Effectiveness of kernel and coat of Moringa oleifera seeds for turbidity removal
Figure 1 shows the efficiency of turbidity removal by seed kernel (naturally dried, sun-dried and oven-dried) and coat of naturally dried seed. The results show that the coat of seed is relatively less efficient and naturally dried seed is more efficient in turbidity removal. Removal to an extent of 36% is possible with dosage of 10 mg coat of seed/L indicating that it has also some turbidity removal potential. This observation is significant as it can be used as coagulant aid due to its larger availability compared to kernel. However, naturally dried seeds can remove turbidity by 90% at a coagulant dosage of 50 mg/L. The comparison of turbidity removal by sun-dried and oven-dried shows that sun-dried seeds are more efficient.
The predominant mechanism for coagulation is adsorption between coagulation active components in Moringa oleifera and particles of suspension to permit inter-particle bridging, and is not charge neutralization (Muyibi and Evison 1995; Ali et al. 2016). The efficiency of adsorption depends on the adsorbent surface area, surface morphology, pore size distribution, polarity and functional groups attached to the adsorbent surface (Ali et al. 2016). This indicates that naturally dried Moringa oleifera seed extract contains more active ingredients with more surface area available for adsorption and inter-particulate bridging (Joseane et al. 2013). The morphological characteristics of the powder of above materials used in the study were carried out through scanning electron microscope (SEM) with different levels of magnification to ascertain extent of porous nature of materials. Figure 2 shows the results of SEM study.
It can be seen that the naturally dried Moringa oleifera seed kernel is amorphous in nature with hips of fine particles which indicate availability of more surface area for adsorption. Sun-dried seed has agglomerated grains resulting in a flocculent structure. Oven-dried seed kernel is relatively more porous compared to these two. Coat is highly porous with reticulated structure providing lesser sites for adsorption.
Further, the time required for drying of pods and its rate also plays a significant role in inducing quality ingredient (protein structure) and surface morphology of seed kernel. Sun-dried and oven-dried were subjected to 1 week and 2 h of drying time, respectively. Complete drying of pods in the tree naturally takes larger time and drying occurs gradually.
This surface morphology study and coagulation study reveal that naturally dried Moringa oleifera seed kernel has greater potential for turbidity removal. Hence, further study is carried out using naturally dried Moringa oleifera seed kernel.
Effect of Strength of Moringa oleifera and Strychnos potatorum extract on turbidity removal
The effect of varied strengths of Moringa oleifera extract was studied using 0.5, 1, 5, 10 and 15% extracts, for water sample of 26 NTU turbidity, and 100 mg/L dose of Moringa oleifera. Figure 3a shows the effect of Moringa oleifera strength on turbidity removal. It can be observed that although removal increases with increase in strength from 0.5 to 15%, the increase is not significant beyond 1%. Hence, 1% strength resulting in 88% removal was used for the further study. Figure 3b shows the turbidity removal with strength 1, 3, 5 and 8% of Aloe Vera for initial turbidity 32 NTU and Aloe Vera dose of 50 mg/L. It can be observed that Aloe Vera as a coagulant is not effective for turbidity removal. However, 1% strength is appropriate for Aloe Vera application as coagulant/coagulant aid. In case of Strychnos potatorum seeds, initial study for deciding dose range revealed that dose of 0.1–1 mg/L is sufficient for optimum turbidity removal. An extract of 0.1% was used considering the practical consideration of dose application. As lower dose application with higher strength extracts is not practicable, the strength effect study is not advisable for Strychnos potatorum seed extract.
Turbidity removal with Moringa oleifera and Strychnos potatorum seeds
Figure 4 shows the residual turbidity after applying varied dose of naturally dried seed kernel of Moringa oleifera for low, medium and high turbid water. It can be observed that for low turbidity water residual turbidity increases with added dose of Moringa oleifera. The colloidal and suspended solids concentration is less for low turbidity water, excess dose results in increase in residual turbidity due to uncombined coagulant, as sufficient particles are not available to combine with coagulant protein. The optimum dose for medium and high turbidity samples was observed to be 50 and 100 mg/L, respectively, with turbidity removal efficiency of 80.95 and 88.57%. This indicates that coagulant dose required increases with initial turbidity for medium and high turbidity water, as more charged sites are necessary for adsorption and chemical bridging. After optimum dose the residual turbidity increases with added dose of coagulant, because of non-availability of sufficient number of particles for bridging with the charged sites of coagulant and turbidity increases due to suspended coagulant matter.
Figure 5 shows the effect of varied dose of Strychnos potatorum on turbidity removal for low, medium and high turbid water. The results show that the removal efficiency increases from high to low turbidity water and for maximum removal optimum dose was observed to be 0.8, 0.6 and 0.2 mg/L for high, medium and low turbidity water, respectively. Thereafter, residual turbidity increases insignificantly. The turbidity removal for low, medium and high turbidity samples was 71.42, 64.28 and 57.14%, respectively.
This study reveals that Moringa oleifera is not effective for low turbidity water, but Strychnos potatorum seeds show good results. For medium, Moringa oleifera is more effective coagulant for high turbidity water, as compared to Strychnos potatorum.
Effect of Aloe Vera as coagulant aid and as polymer with ballasting agents in turbidity removal
The efforts were taken to improve the efficiency of coagulation process by using Aloe Vera as a natural coagulant aid with Moringa oleifera and Strychnos potatorum as natural coagulants. Table 1 shows the comparison of optimum dosage of Moringa oleifera, when used with and without Aloe Vera. The results show that Aloe Vera is more effective as coagulant aid when used with Moringa oleifera for low turbidity water and relatively less effective for medium and high turbidity water. Aloe Vera acts as a polymer which possesses charged sites, to which micro-flocs formed with Moringa oleifera get attached, and large settleable flocs are formed.
This table also gives the optimum combination of Moringa oleifera, Aloe Vera and micro-sand for turbidity removal. It can be seen that the use of ballasting agent (micro-sand) with Aloe Vera has enhanced the turbidity removal. The dosage of Moringa oleifera is also reduced for medium and high turbidity water. This is due to improved settling conditions with increase in weight of flocs. Addition of polymer leads to branched floc formation and sand provides surface area for floc formation. The micro-flocs formed lead to the formation of macro-flocs by attaching with micro-sand. The physical attachment and enmeshment followed by fast settling macro-flocs are the causes of enhanced turbidity removal.
Table 2 shows the comparison of optimum dosages of Strychnos potatorum with and without AV. The combination of Strychnos potatorum and AV did not give good results as compared with MO and AV. This may be due to AV, which bridges with some of the charged sites on coagulant protein, making them unavailable to the colloidal particles. At higher concentration of colloidal and suspended matter, the removal is more due to greater opportunity for contact to form large and readily settleable flocs. This table also shows that the use of micro-sand and AV as polymer with Strychnos potatorum improves turbidity removal for medium and high turbidity water, as compared with Strychnos potatorum alone. Although removal improves by this combination, post-treatment is necessary to reduce the turbidity below 1 NTU.
Table 3 shows the results of ballasted coagulation with MO, AV and powdered activated carbon (PAC). It can be seen that the use of PAC as ballasting agent with MO and AV has resulted in residual turbidity of 1 NTU or below 1 NTU for all types of turbid water. The larger surface area and surface texture of PAC particles, coating with Aloe Vera, formation of micro-flocs induced by MO with nucleus of PAC, and adsorption of colloidal particles contribute to improved turbidity removal. The results of ballasted coagulation by SP, AV and PAC given in Table 1 show enhanced turbidity removal but resulting residual turbidity is 2 NTU or below 2 NTU.
Effect on settling time in ballasted coagulation
Figure 6 shows the effect on settling time in ballasted coagulation with micro-sand and PAC. The residual turbidity was measured after coagulation–flocculation for 10, 20 and 30 min of settling. It can be observed that the use of ballasting agents micro-sand or PAC reduces settling time from 30 to 10 min for obtaining residual turbidity below 4 NTU, as compared with MO, and MO + AV for all three categories of turbid water. Dose of MO increased the residual turbidity for low turbidity water, hence the effect on settling time was not considered.
Figure 6 also shows that the use of micro-sand and PAC reduces settling time to 20 min, and 10 min, respectively, for low turbidity water to reduce residual turbidity to 3 NTU. The settling time was reduced to 10 min for medium and high turbidity water by both micro-sand and PAC for obtaining residual turbidity of 6 NTU. Settling time was observed to be 30 min without the use of ballasting agents.
Addition of polymer in treatment leads to branched floc formation and addition of sand/PAC increases weight of the flocculated particles leading to faster settling of suspended solids. The polymer binds the sand/PAC to the floc, thereby avoiding dispersed floc. The reduction in settling time is significant as high-rate clarifiers such as tube/plate/lamella settlers can be used in separation.
Effect of Aloe Vera on dosages of MO and SP in ballasted coagulation
Figure 7a shows reduction in MO dose, with AV and ballasting agents. It can be seen that the dose of MO was reduced to 30 mg/L, 10 mg/L and 10 mg/L for high, medium and low turbidity water for obtaining residual turbidity of 4 NTU, respectively. The dose reduction is due to improved coagulation, flocculation and settling of particles, with the aid of polymer and ballasting agents.
Figure 7b shows reduction in SP dose, when used in combination with AV and ballasting agents to reduce the residual turbidity below 4 NTU. The ballasting agents are effective with coagulant and polymer, even for low turbidity water. The study shows that ballasting agents reduce the time and quantity of coagulant required for water treatment.