Photo Catalytic Degradation of Organic Pollutants in Dairy Effluent

8/3/2019 Photo Catalytic Degradation of Organic Pollutants in Dairy Effluent

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Photocatalytic degradation of organic pollutants in dairy effluent

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Abstract

The present study was aimed to treat the dairy wastewater by photocatalytic

oxidation methods. The photocatalytic treatment was carried out in a laboratory scale

batch reactor with a working volume of 650mL. The main objective of the study is to

evaluate the COD reduction of the effluent by varying parameters such as different

catalysts (ZnO and TiO 2), pH, effluent initial concentrations and light source (UV lamp

and Mercury lamp). The photocatalytic oxidation using TiO 2 removed higher percentage

of COD than ZnO in the dairy effluent. The findings suggest that photo catalytic

oxidation would be a promising alternative for the treatment of dairy wastewater.

Keywords: photocatalytic; dairy; COD; TiO 2

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1. Introduction

Dairy industries release large quantities of wastewater often the order of thousand

cubic meters /day [1]. The dairy wastewater is similar to most other agro-industries

wastewaters, characterized by high biological oxygen demand (BOD) and chemical

oxygen demand (COD) concentrations representing their high organic content [2] High

concentration of organic matter in dairy wastewater causes pollution problems to

surroundings [3]. Aerobic methods tried to treat and dispose dairy wastewater [4] have

been less efficient, mainly due to bulking and excessive growth of biomass [5].

Installation cost of aerobic treatment systems are very high and require considerable input

energy for aeration. Anaerobic method for the treatment of dairy wastewater is attracting

the attention of researchers because of the presence of high organic content in the waste,low energy requirement of the process, lesser sludge production and generation of fuel in

the form of methane [6]. It is reported that the efficiency of high rate anaerobic reactors

can be improved by restricting the supporting material to the top 25–30% of the reactor

volume [7]. Such a modification would further help to realize the advantages of both

fixed film and up flow sludge blanket treatment. This kind of reactor, often called the

hybrid anaerobic reactor has been reported to be more stable for the treatment of a series

of soluble or partially soluble wastewater [8]. Over the years, hybrid reactors have been

used to treat wastewaters from sugar industry, sago, distilleries [9] and domestic sectors

[10]. The elimination of organics present in wastewater has been investigated using

chemical and photochemical processes with varying successes [11]. Photocatalysis using

semiconductors is a subject of increasing interest which has been extensively performed

worldwide to find solutions for wastewater treatment, since the discovery of ‘Honda–

Fujishima Effect’ three decades ago. In these processes, the oxidation occurs through an

attack of OH_, which has a rate constant billions of times higher than normal rate

constants, using air as the oxidant. Among the semiconductors reported so far,

outstanding stability and oxidative power makes TiO 2, the best semiconductor

photocatalyst for environmental remediation and energy conversion processes [12]. The

UV radiation required for the photocatalytic processes can be obtained from artificial

sources or the sun. There is a significant economic incentive for solar light based



photocatalytic degradations. Recently [13] successfully reported the treatment of

wastewater combining UASB technology and advanced oxidation processes (AOP’s) to

treat food processing wastewaters. The solar photocatalytic treatment has the advantage

to achieve additional destruction of organics in the wastewater but it is effective only to

low strength wastewater [14]. Considering this fact it was employed after the primary

anaerobic treatment. The treatment of wastewater using UV radiation holds promise for

most part of the Asia [15]. Keeping it in mind, in the present study an attempt has been

made for treating dairy wastewater.

2. Materials and methods.

The effluent is collected from avin milk dairy, Karaikudi. The organics

concentration in the dairy wastewater was adjusted to the desired level by diluting the

feed water using distilled water. The characteristics of collected effluent is shown table.1

2.1 UV lamp setup:The lab scale photo catalytic reactor is constructed from stainless steel. The

maximum loading capacity of the reactor is 500ml. A PVC frame was mounted on top of

the reactor. The PVC frame had provisions for inserting the UV lamp. The exposed

surface of the UV lamp was housed in PVC tube.

2.2 Mercury lamp set up:



The lab scale set up for photo catalytic oxidation is a chamber fitted with 16 watt

mercury lamp. The chamber has four reflecting surfaces. 400 ml of effluent was taken in

a glass beaker and placed over a magnetic stirrer inside the chamber. The chamber was

covered with a wooden board. Provisions were made for collection of samples.

2.3 Analysis of COD

This test is highly useful to find out the pollution strength of industrial effluents

and sewage. Chemical oxygen as the name implies is the oxygen requirement of a sample

for oxidation of organic and inorganic matter. COD is generally considered as the oxygen

equivalent of the amount of organic matter utilizable by potassium dichromate(K 2Cr 2O7).

The organic matter of the sample is oxidized to water, carbon di-oxide and ammonia

by reflux ion with a known excess of potassium di chromate in a 50% sulphuric acid

solution. The excess dichromate is titrated with a standard solution of ferrous ammoniumsulphate solution

3. Results and discussion

3.1 Effect of pH and initial concentration.

Solution pH is an important variable in the evaluation of aqueous-phase mediated

photocatalytic reactions. It influences adsorption and dissociation of the substrate,

catalyst surface charge, oxidation potential of the valence band and other

physicochemical properties of the system. Hence experiments were conducted to study

the impact of pH on the rate of photocatalytic degradation by keeping the catalysts TiO 2

and ZnO constant (300 mg/L) and varying the pH of the anaerobically treated dairy

wastewater 4.5 and 7.5. From the results we found that higher COD removal achieved at

the pH of 7.5.

3.2 Effect of catalysts.

In this study two types of catalysts TiO 2 and ZnO were used for the photocatalytic

degradation of dairy effluent. Titanium dioxide in the anatase form appears to be the most

photo-active and the most practical of the semiconductors for widespread environmental

application such as water purification, wastewater treatment, hazardous waste control, air

purification, and water disinfection. ZnO appears to be a suitable alternative to TiO 2;

however ZnO is unstable with respect to incongruous dissolution to yield Zn(OH) 2 on the

ZnO particle surfaces and thus leading to catalyst inactivation over time and good results



have been obtained but their applications remain limited only by pH. From the results it

was found that TiO 2 shows better COD removal than ZnO.

3.3 Photocatalytic degradation of dairy effluent

Photocatalytic oxidation utilizes ultraviolet or near-ultraviolet radiation to

promote electrons from the valence band into the conduction band of a titanium dioxide

semiconductor. Destruction of organic compounds takes place through reactions with

molecular oxygen or through reactions with hydroxyl radicals and super-oxide ions

formed after the initial production of highly reactive electron and hole pairs. As the

electrons move form valence band to conduction band TiO 2 generates electron hole-pairs

to produces hydroxyl radicals. The hydroxyl radicals react with organic compounds to

produce water and a reactive organic radical. This organic radical combines with oxygen

to form peroxides which are released into the atmosphere. Figures 1 & 2 shows clearlythe photocatalytic degradation of dairy effluent. From this figure we found that 76.6%

and 66.6 percentage of cod reduction was obtained by TiO 2 and ZnO respectively. During

photocatalytic oxidation all the pollutants were converted to CO 2.

Fig.1. Photo degradation using TiO 2 (COD reduction: 76.6%)



Fig.2. Photo degradation using ZnO (COD reduction: 66.6%)

Conclusions.

The photocatalytic treatment was carried out in a laboratory scale batch reactor

evaluated the COD reduction of the dairy effluent by varying parameters such as

different catalysts (ZnO and TiO 2), effluent initial concentrations and light source (UV

lamp and Mercury lamp). The working pH and catalyst loading for the photochemical

oxidation were 7.5 and 5gpL, respectively. The photocatalytic oxidation using TiO 2

showed higher percentage remival of COD than ZnO in the dairy effluent. The findings

suggest that photo catalytic oxidation would be a promising alternative for the treatment

of dairy wastewater. From the results it was found that Photo catalytic oxidation with

UV lamp gave a maximum COD removal of 77% at a pH of 7.5 in 4 hrs.

Acknowledgement

Authors thank Ministry of Environment and Forests for the financial support to

carry out the work under Project No. GAP 23/07 and the General Manager, Aavin milk

processing company, Karaikudi, Tamilnadu for providing the dairy effluent.

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Photo Catalytic Degradation of Organic Pollutants in Dairy Effluent