Wastewater Treatment Using Membrane Filtration

Introduction

One major application of the membrane technology involves treatment of wastewater (Baker, 2004; Bourgeous, Darby and Tchobanoglous, 2001). The practice has become popular over the recent past as new laws and developments in membrane technology continue to set new standards for practising. Specifically, the membrane bioreactor (MBR) approach for wastewater treatment has gained wide recognition and application as it is an alternative to the traditional activated sludge methodology (Chang, Le-Clech Jefferson and Judd, 2002). The technology exploits the ability of the membrane to retain all microorganisms completely during treatment of wastewater. However, there is a critical limitation with the application of membrane technology in wastewater treatment. The process relies on membrane fouling, which is a process of accumulating biosolid cake coating onto the membrane surface to reduce the permeate flux. This is a critique of an article, Wastewater treatment using membrane filtration—effect of biosolids concentration on cake resistance by Chang and Kim (2005).

Purpose of the study

Chang and Kim conducted a study to investigate “the effect of biosolid concentration on filtration characteristics in wastewater treatment system, by using and comparing two different treatment processes i.e., MBR and tertiary wastewater treatment with membranes” (Chang and Kim, 2005). Therefore, the researchers defined the scope and purpose of their study.

Conceptual framework

From the review of available studies, the researchers noted that many studies about membrane fouling concentrated on showing how factors like dissolved organic carbon (COD), viscosity, and temperature affected membrane fouling (Ishiguro, Imai and Sawada, 1994; Sato and Ishii, 1991; Defrance and Jaffrin, 1999). Consequently, Chang and Kim (2005) noted that few studies focused on biosolid concentrate. They attributed this possibility to a claim that “all the microbial biosolids could be completely rejected by the membrane” (Chang and Kim, 2005).

The researchers based their study on mixed liquor of suspended solids (concentration). Studies have shown that MLSS had direct effects on the cake layer deposit on the surface of the membrane. They noted that the deposit cake layer on the surface of the membrane had a significant impact on the permeate flux. Thus, a study could reveal if biosolid deposits on a cake layer had impacts on membrane fouling and if it could provide a method of controlling membrane fouling.

The available literature also indicated that membrane fouling in MBR and subsequent treatment of wastewater with membrane filtration were linked to physiochemical contacts between the membrane and biofluids. Any contact between the membrane surface and chemical suspension results into immediate deposition of biosolids on the surface, which leads to a decline flux. The cake layer can be easily separated from the membrane surface by using a suitable washing technique and procedure. Hence, this is a reversible fouling. Conversely, there is also irreversible fouling or internal fouling, which results from adsorption of dissolved chemicals into the membrane through its pores. However, when there is blockage of pores, then the process is irreversible and may only be separated through chemical processes.

Therefore, Chang and Kim (2005) based their study on a sound theoretical background supported with relevant past studies in the field of membrane technology.

Methodology

The researchers clearly illustrated all materials and methods applied in the study. For instance, they demonstrated the process of development of the activated sludge. They noted that the process involved the use of artificial wastewater, which was prepared by using sterile and concentrated mixture. This was the main solution used throughout the study. The researchers used the process as illustrated by Chang, Lee, and Ahn (1999).

Another procedure involved membranes and filtration. In this process, Chang and Kim (2005) applied two types of filtration methods. The first one involved a “batch type dead-end filtration while the second one was a continuous fed submerged membrane system” (Chang and Kim, 2005).

Another process in the study method was analytical procedure. The researchers based their MLSS concentration on the Standard Method provided by the American Public Health Association (APHA), the American Water Works Association (AWWA), and the Water Environment Federation (WEF) (Greenberg, 1992). In addition, the analysis of COD involved the observation of the US EPA approved methodology. They used the Hach Laboratory technique 800 during the procedure for preparation of the MLSS concentration and a laser scattering size analyser was used in floc size analysis of the sludge.

Results and Discussion

The researchers adopted a two-phase analysis of data for convenience. The first part involved the analysis of the dead-end typed filtration of activated sludge. Analysis entailed adjusting time to allow various MLSS concentrations with activated sludge mixtures to settle. They also withdrew some elements of the liquid on the surface and exposed it to air. Chang and Kim noted that there was a reduction in flux in the process of ultrafiltration because of the adjusted settling time. This allowed researchers to note the change in MLSS. Therefore, analysis of fouling trend involved quantitative method in which they calculated different resistances, ratio, and the exact specific cake resistance and then presented results as a summary.

The second analysis involved continuous-fed submerged MBR and tertiary treatment with membrane. The researchers expected an improved performance in flux relative to the MBR procedure because the resultant waste matter had already undergone clarification and had low concentration of MLSS. Analysis also involved a comparison between tertiary treatment and lab-scale and the submerged MBR based on operation time.

Therefore, one can observe that all major processes that required analyses were presented a manner that was simple to understand for the study audience. Hence, other researchers can easily follow the analysis processes in this study.

Conclusion

From the study results, Chang and Kim concluded that cake resistance declined with the decrease in concentration of MLSS. They also observed that certain cake resistance increased when there was a decline in MLSS concentration. That is, the two exhibited opposite patterns despite their similar theoretical underpinnings. This showed that specific cake resistance was not a suitable standard for determining the level of cake fouling, particularly with low concentrated MSLL. In tertiary treatment, the researchers expected good results in flux outcomes relative to MBR process. In both cases of tertiary treatment and submerged MBR, there was no observable change in performance of flux. The secondary waste material had small particles, but the flux did not improve relative to MBR even under lower concentration of MLSS. Therefore, from a perspective of membrane fouling and wastewater treatment, tertiary wastewater treatment with membrane does not offer any additional advantages over the MBR process.

Chang and Kim (2005) provided conclusion for their study and indicated that tertiary wastewater treatment was not beneficial. This is an implication for industry professionals to adopt effective processes and eliminate unnecessary procedures in wastewater treatment.

References

Baker, R. W. (2004). Membrane Technology and Applications (2nd ed.). West Sussex: John Wiley & Sons.

Bourgeous, N., Darby, L., and Tchobanoglous, T. (2001). Ultrafiltration of wastewater: effects of particles, mode of operation, and backwash effectiveness. Water Res., 35, 77-90.

Chang, I-S., and Kim, S-N. (2005). Wastewater treatment using membrane filtration—effect of biosolids concentration on cake resistance. Process Biochemistry, 40, 1307–1314.    doi:10.1016/j.procbio.2004.06.019.

Chang, S., Le-Clech, P., Jefferson, B., and Judd, S. (2002). Membrane fouling in membrane bioreactors for wastewater treatment. Journal of Environmental Engineering, 128(11), 1018–29.

Chang, S., Lee, H., and Ahn, K. (1999). Membrane filtration characteristics in membrane coupled activated sludge system: the effect of floc structure on membrane fouling. Sep Sci Technology, 34, 1743–58.

Defrance, L., and Jaffrin, Y. (1999). Comparison between filtrations at fixed transmembrane pressure and fixed permeate flux: application to a membrane bioreactor used for wastewater treatment. Journal of Membrane Sci., 152, 203–10.

Greenberg, A. E. (1992). Standard Methods for the examination of water and wastewater (18th ed.). Washington, DC: APHA-AWWA-WEF.

Ishiguro, K., Imai, K., and Sawada, S. (1994). Effects of biological treatment conditions on permeate flux of UF membrane in a membrane/activated sludge wastewater treatment system. Desalination, 98, 119–26.

Sato, T., and Ishii, Y. (1991). Effects of activated sludge properties on water flux of ultrafiltration membrane used for human excrement treatment. Water Sci Technology, 23, 1601–8.