Results and discussions

Poultry processing wastewater is highly laden with organic matters. Among the various contaminants present in poultry wastewater, almost 35% corresponds to the large floating debris resulting from the agglomeration of grease and fat, while the major fraction (around

55%) corresponds to suspended solids containing lipids, proteins, pathogenic microbes, etc.

These suspended solids are the prime reason for the formation of opaque haze in poultry wastewater. The rest 5-10% of contaminants are contributed by particles sticking to the emulsified globules present in the wastewater (Avula et al., 2009). Hence, the characteristics of the poultry wastewater collected for testing the filtration performance of the membrane were also evaluated and the results are mentioned in Table 4.2. It needs to mention that the BOD5

value of the raw wastewater mentioned in Table 4.2 was calculated using the ratio BOD5/COD

= 0.6. This ratio has been extensively used in many research works related to poultry slaughterhouse wastewater to calculate the BOD5 values from the experimentally obtained COD values (Shih and Kozink, 1980; Whitehead, 1976).

Table 4.2 Characteristics of poultry slaughterhouse wastewater (feed)

Parameter Values

pH 6.19 ± 0.07

Total suspended solids (mg/L) 414.50 ± 69.50

Turbidity (NTU) 20.70

COD (mg/L) 271.95

BOD5 (mg/L) 163.17

The poultry wastewater was passed through the membrane module at five different pressures, keeping all the other parameters constant, as mentioned in Table 4.1. It is evident from Fig. 4.2 that the permeate flux increases with an increase in the applied pressure. The pressure being the driving force in this membrane separation process, its increasing value causes increased driving force, thus leading to a higher flow of permeate (Zhang et al., 1997). However, the flux declines with time, which may be due to the resistance offered by pore blocking and formation of the cake layer. In the initial period of filtration, a rapid decrease in the flux is noticed due to pore blocking of foulant particles. However, the fouling caused by the formation of cake layer on the membrane surface due to the deposition of foulants makes the decline of the permeate

flux gradual. The fouling occurred due to cake layer formation over the membrane surface is regarded as the reversible form of membrane fouling (Rodrigues and Fernandes, 2012).

Fig. 4.2 Permeate flux collected for the duration of three hours at pressures 207, 276,345, 414 and 483 kPa (Cross flow rate: 11.11×10^-6 m³/s)

It was reported in the literature that the proteins present in poultry wastewater do not possess a fixed conformation and hence, they get destabilized on the application of high shear, leading to membrane fouling and subsequent flux decline (Lo et al., 2015). As evidenced, the flux decline is faster and more pronounced at higher pressures as a higher shear rate breaks the organic matters and leads to enhanced pore blocking (Kumar et al., 2016). At lower pressures, insufficient shear rate makes the breaking of foulant particles quite difficult, thus resisting pore blocking. Hence, the decline in flux at 207 kPa is quite insignificant as compared to other pressures (Ren et al., 2019). It can be said that the fouling at higher pressures is attributed to the combined effect of cake formation as well as pore blocking, while the impact of pore blocking gets diminished at lower pressures.

If the question comes for choosing an optimum pressure at which the system needs to be operated, 414 kPa can be considered as it not only gives 100% rejection, but also gives high

flux with lesser flux decline. Pressure 483 kPa cannot be taken as the optimum operating pressure, albeit providing a higher flux, owing to its higher flux decline during the operation time. The obtained permeate is clear in colour with zero turbidity value for all five pressures, as depicted in Fig. 4.3. As evident from Fig. 4.4, complete removal of COD and TSS is achieved through this membrane. The larger size of the particulate matters present in the poultry slaughterhouse wastewater is the prime reason for achieving 100% reduction in COD, TSS and turbidity of the processed water. The major contaminants of poultry slaughterhouse wastewater are the suspended solids having the size in the range of 20-50 µm. As we mentioned earlier that the fats and globules present in the wastewater are in the form of large agglomerates and debris that float on the water surface and hence, these can be easily removable. The rest of the contaminants, which are found to be bound with emulsified globules, have sizes of around 5 mm (Avula et al., 2009). It is noteworthy to mention that the pore size of the membrane (0.133 µm) is lower than the sizes of the above-mentioned organic matters present in poultry wastewater, which generally contribute to COD and TSS content. Hence, the membrane could achieve the complete removal of them via size exclusion principle.

As previously mentioned, as filtration proceeds, the formation of a cake layer takes place on the membrane surface by the foulants present in poultry slaughterhouse wastewater. In order to investigate the formation of cake layer on the membrane surface, after end of microfiltration, the inner surface of the membrane, where feed was passed through it, was screened under Field Emission Scanning Electron Microscope (FESEM), as reported in Fig. 4.5. As evident from Fig. 4.5, particles present in the wastewater were deposited on the membrane, which reduces the porous structure of the membrane during filtration. It is worth mentioning that the permeate water collected through the process satisfies the COD and TSS norms provided by Central Pollution Control Board, India, for its reuse as well as discharge into the environment. It also justifies the regulation given by the World Health Organisation in terms of turbidity value that

can even be accepted for drinking purposes (Tatwawadi, 2017; WHO, 2017). Looking at the high quality of permeate produced, it can be inferred that the water is safe to discharge into the environment and can also be reused in the poultry industry itself to tackle the stringent water demand.

Fig. 4.3 Poultry slaughterhouse wastewater before and after membrane filtration

Fig. 4.4 COD and TSS reduction after membrane filtration at all five different pressures

Fig. 4.5 Inner surface of the membrane as observed under FESEM (a) before and (b) after poultry slaughterhouse wastewater treatment