Aristotle University of Thessaloniki1
Given the worldwide water shortage and future needs, the debate on the utilization of recycled wastewater is gaining more and more attention. Reclaiming and purifying water for subsequent reuse (e.g. irrigation) reduces an overall demand for water, whereas, enhances stability of water supply and leads to the protection of vulnerable ecosystems.
One major source of highly reclaimable water, particularly important for water-stressed nations, is the urban wastewater generated from washing activities in a household (laundry, showers, kitchen sinks, hand basins etc.) which is generally referred to as "greywater" and accounts for up to 75% of domestic wastewater [1, 2]. With this in view, treatment, with subsequent recycle and reuse constitutes a very attractive alternative from an environmentally sustainable perspective, as it offers the potential to substantially reduce domestic water demand in water scarce regions.
The aim of this study is to investigate the detoxification of greywater effluent by means of TiO2-mediated heterogeneous photocatalysis and more specifically to evaluate the effect of various operating conditions i.e. catalyst dosage and effluent pH on process efficiency.
Material and Methods
The catalyst used was Aeroxide P-25 TiO2 (Evonik), a benchmark photo catalyst with high efficacy. Greywater was synthetically produced in the laboratory, taking into consideration the physical, chemical, and biological characteristics of actual greywater samples taken during an-one month sampling campaign; the laboratory-simulated greywater represents the overall characteristics greywater from very high strength to low strength in terms of water quality parameters e.g. DOC, COD, BOD, N-NO3, P-PO4 and TSS. The sampling campaign was applied at three different sources of greywater i.e. hand basin (COD= 250Â±140 mg/L; pH=7Â±0.14), shower (COD=347Â±274 mg/L; pH=7Â±0.17) and washing machine (COD=3343Â±1709 mg/L; pH=6.7Â±0.98). A simulated effluent was produced composed of laundry detergent, toothpaste, shampoos and soaps reaching a final COD value 1250 mg/L and pH around 7.
Photocatalytic experiments were carried out in a thermostated pyrex cell of 500 mL capacity. The reaction vessel was fitted centrally with either a UV-A or Visible, 9W light of identical dimensions and geometry. The reactor was covered with a black cloth to avoid interactions with ambient light. Effluent mineralization was followed by measuring the chemical oxygen demand (COD) according to the potassium dichromate method (Hach Europe, Belgium). Color abatement during the photocatalytic oxidation was monitored on Shimadzu UV -- 1700 UV--vis spectrophotometer by scanning the sample absorbance in the 200--800nm band. Anionic surfactants concentration was measured according to Standard Methods . Changes in effluent pH and conductivity after electrolytic treatment were determined with a Crison GLP 21 pH meter and a Mettler Toledo conductivity meter, respectively.
Results and Conclusions
Preliminary experiments performed in the absence of catalyst (i.e. photolysis) verify the high photochemical stability of greywater at inherent pH (6.7-8.3). DOC removal increased with the increasing catalyst dosage from 0.5 g/L up to 5 g/L, although no significant photocatalytic activity was observed (i.e. less than 20%); most of the DOC abatement observed was due to organic adsorption onto catalyst particles. Initially, DOC remained constant and then there was a gradual decrease, due to the early transformation of the parent compounds into intermediates that in time prove more amenable to chemical oxidation. The low efficacy of the photocatalytic process can be attributed to the high COD content of the reaction system. Lowering the pH of the effluent (e.g. pH = 4) results in higher process efficacies as the adsorption extent is enhanced. In all cases high anionic surfactant removal ratios were observed.
This work was implemented under the REGREW (PE10 (2472)) project within the framework of the Action 'Supporting Postdoctoral Researchers' of the Operational Program Education and Lifelong Learning (Action's Beneficiary: General Secretariat for Research and Technology) and is co-financed by the European Social Fund (ESF) and the Greek State.
1. Eriksson, E., Auffarth, K., Eilersen, A-M., Henze, M. and Ledin, A. (2003) Household chemicals and personal care products as sources for xenobiotic organic compounds in grey wastewater. Water SA 29 (2) 135-146
2. Eriksson, E., Auffarth, K., Henze, M., Ledin, A. (2002) Characteristics of grey wastewater, Urban Water 4 (1) 85--104
3. WPCF, AWWA and APHA, Standard Methods for the Examination of Water and Wastewater, 20th Ed, American Public Health Association (APHA), Washington DC, 1998.