2.4. Analytical methods
To determine total cell count the samples were serially diluted with sterile saline solution (0.85% w/v NaCl). The appropriately diluted samples (0.1 ml) were plated and incubated at 30 °C for 24 h to form fully developed colonies. The cell count was estimated by counting colonies grown on nutrient agar medium using a haemacytometer [25]. For all counts, the average of at least three replicate plates was used for each tested dilution. In order to establish the reliability and reproducibility of the plate count technique, 10 independent samples were drawn (at the same time) from the shake flask experiment and were serially diluted and plated. Each dilution was plated in three different plates and the colonies were counted and the standard deviation was calculated using Microsoft Excel's built-in STDEV function (“non-biased” or “n−1” method). The standard deviation was 6%.
The wastewater characteristics such as pH, suspended solids (SS), MLSS, COD ammonia were analyzed following standard methods [26] and the cyanide content was analyzed using a modified ninhydrin method as follows [27]: wastewater sample was centrifuged at 6200 rpm, for 5 min. Then, 2 ml of supernatant was mixed with 2 ml ninhydrin solution. After 10 min incubation at room temperature, the absorbance was measured at 485 nm using spectrophotometer (UV-1201 Shimadzu). Samples were analyzed in triplicates. Standard deviations were less than 5% of the average.
3. Results and discussion
3.1. Characteristics of cassava mill wastewater
Table 1 shows the composition of cassava wastewater. The high organic content of 16,000 mg l−1 in the wastewater found in this study is in agreement with those found typically in large scale starch processing industries [28] and [29]. These organic pollutants originate from the washing and extracting processes in the tapioca industry [30] and [31]. In order to produce 1 ton of starch, a tapioca processing factory discharges about 12 m3 of wastewater containing 6125–13,500 mg l−1 (COD), 1466–7600 mg l−1 (SS) and pH 4.5–5.0 [32] and [33]. These values indicate that the wastewater is highly biodegradable and therefore, there is no need for the supply of external organic carbon source for cyanide-degrading microoganisms. The wastewater pH was low (pH 5.5) and probably arises from the acidification process of starch due to organic acid production [34]. The low carbohydrate content (16 mg l−1) of the wastewater was in agreement with this observation. A high cyanide concentration of 86 mg l−1 was observed in this study comparing the other cassava mill wastewater studies (Table 1) and this could arise from the different variety of cassava that is being treated. Elsewhere, cyanide concentration in cassava mill wastewater was reported contain up to 200 mg l−1 depending on the cyanoglycoside content of the cassava varieties
2.4. Analytical methods
To determine total cell count the samples were serially diluted with sterile saline solution (0.85% w/v NaCl). The appropriately diluted samples (0.1 ml) were plated and incubated at 30 °C for 24 h to form fully developed colonies. The cell count was estimated by counting colonies grown on nutrient agar medium using a haemacytometer [25]. For all counts, the average of at least three replicate plates was used for each tested dilution. In order to establish the reliability and reproducibility of the plate count technique, 10 independent samples were drawn (at the same time) from the shake flask experiment and were serially diluted and plated. Each dilution was plated in three different plates and the colonies were counted and the standard deviation was calculated using Microsoft Excel's built-in STDEV function (“non-biased” or “n−1” method). The standard deviation was 6%.
The wastewater characteristics such as pH, suspended solids (SS), MLSS, COD ammonia were analyzed following standard methods [26] and the cyanide content was analyzed using a modified ninhydrin method as follows [27]: wastewater sample was centrifuged at 6200 rpm, for 5 min. Then, 2 ml of supernatant was mixed with 2 ml ninhydrin solution. After 10 min incubation at room temperature, the absorbance was measured at 485 nm using spectrophotometer (UV-1201 Shimadzu). Samples were analyzed in triplicates. Standard deviations were less than 5% of the average.
3. Results and discussion
3.1. Characteristics of cassava mill wastewater
Table 1 shows the composition of cassava wastewater. The high organic content of 16,000 mg l−1 in the wastewater found in this study is in agreement with those found typically in large scale starch processing industries [28] and [29]. These organic pollutants originate from the washing and extracting processes in the tapioca industry [30] and [31]. In order to produce 1 ton of starch, a tapioca processing factory discharges about 12 m3 of wastewater containing 6125–13,500 mg l−1 (COD), 1466–7600 mg l−1 (SS) and pH 4.5–5.0 [32] and [33]. These values indicate that the wastewater is highly biodegradable and therefore, there is no need for the supply of external organic carbon source for cyanide-degrading microoganisms. The wastewater pH was low (pH 5.5) and probably arises from the acidification process of starch due to organic acid production [34]. The low carbohydrate content (16 mg l−1) of the wastewater was in agreement with this observation. A high cyanide concentration of 86 mg l−1 was observed in this study comparing the other cassava mill wastewater studies (Table 1) and this could arise from the different variety of cassava that is being treated. Elsewhere, cyanide concentration in cassava mill wastewater was reported contain up to 200 mg l−1 depending on the cyanoglycoside content of the cassava varieties
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