1. Introduction
Dairy industries consume huge amounts of water, accounting for 33.96% of water consumption in all food industries [
1]. Water is consumed in different ways in dairy industries. Examples include; as an ingredient, in clean in place (CIP), as boiler feed, in cooling tower operation, etc. Among these operations, CIP accounts for 38% of the total water consumption in the dairy industry [
2]. Due to the large aforementioned point uses of water, the dairy industry generates 0.2 L to 10 L of effluent per L of processed milk [
3].
Dairy wastewaters are characterized by high chemical oxygen demand (COD) due to high organic content caused by the presence of fats, proteins and carbohydrates [
4]. This high nutrient content in the dairy wastewater is due to dumping dairy products down the drain and cleaning processed equipment and pipes [
5,
6]. When the highly nutritive effluent from the dairy industry is not treated and dumped into rivers, it causes eutrophication by organic, nitrogen and phosphorous compounds [
7,
8]. In most municipalities where there is no river close by, the dairy wastewater is sent to a municipal wastewater treatment (MWWT) plant for further treatment, before being discharged into other bodies of water. This gives a final safe layer of treatment. Depending upon the type and volume of products manufactured, municipal wastewater treatment plants set limits on certain water quality parameters (COD, BOD, fats, oils, total solids, etc.). If industries discharge water to the MWWT plant that is over the limit, they receive penalties in terms of surcharges [
9]. Hence, treating the wastewater before discharging either into a river or to the municipal treatment plant is necessary to avoid serious environmental and financial impacts. The type and volume of dairy products manufactured varies from one industry to another, causing high variability in the nature of dairy wastewater, making it difficult to choose a particular wastewater treatment method. Current treatment methods for dairy wastewater are aerobic treatment, anaerobic treatment, dissolved air floatation (DAF), activated sludge, clarification, sand bio filters, membrane filtration, advanced oxidation processes, coagulation/electrocoagulation, moving bed biofilm reactors, membrane bioreactors and other treatments. Combined treatments are most effective to tackle the heterogeneity of the wastewater [
10,
11]. DAF is the most commonly used treatment method for dairy wastewater [
12].
CIP consumes the most water in the dairy industry, and thus, also generates the highest volume of effluent among all dairy operations. High concentrations of dissolved salts and high pH in the dairy effluent is due to CIP operation [
11]. Therefore, depending upon the operation going on in the dairy plant, the influent to the wastewater treatment plant varies. To tackle this high heterogeneity in the feed, big equilibrium tanks (EQ) are used. These huge tanks collect all the wastewater from the dairy plant over a long period and mix them using pumps to produce a certain degree of homogeneity in the feed to the wastewater treatment plants.
A dairy industry in Ohio utilizes dissolved air floatation (DAF) as its wastewater treatment method. This method uses coagulants, flocculants and air bubbles to remove suspended particles from the water. Coagulants like polyaluminum chloride are used todestabilize the suspended particles, and flocculants—like acrylamides— aggregate these destabilized particles into big clusters. These clusters adhere to the surface of the air bubbles and rise to the top of the DAF where it is skimmed off. This type of treatment method is effective against treating wastewaters containing high fat, oil and greases and suspended solids. The disadvantage of this treatment is that it is a very chemical-intensive process. This dairy industry had a COD discharge limit of 1200 ppm to the municipality and it received surcharges, since its wastewater contained significant dissolved solids and the DAF was not effective against treating them. Thus, there was a need for another wastewater treatment.
Membrane filtration is not only used as a wastewater treatment method in dairy industries [
13], but also for the reclamation and reuse of water [
14,
15], whey fractionation [
16], recovery of cleaning solutions [
17] and other purposes [
18]. In cross flow membrane filtration, the feed is more highly pressurized than the osmotic pressure across a semipermeable membrane, and the particles that can pass through the membrane pores come out through the circumference as permeates, while the other particles are retained and come out the other end as retentate. Membranes will foul with time due to the accumulation of particles on the pores, and thus, the permeate flux reduces. To recover lost flux, a membrane has to be cleaned from time to time, and, after certain a number if cleanings, replaced [
19]. With technology, membranes are manufactured to reduce fouling effects and withstand a wide range of pH and temperatures of the feed. The use of membranes for wastewater treatment is increasing due to the availability of low cost and more versatile pore sized membranes. Due to the various advantages of this treatment method, the influence of membrane filtration on dairy waste streams was studied.
The overall objective for this investigation was to evaluate the use of membrane filtration for the management of the wastewater stream from a dairy manufacturing operation. The specific objectives were; (1) to compare the effectiveness of dissolved air floatation (DAF), membrane filtration and combined DAF and membrane filtration for reducing the COD of the waste water stream, (2) to select the appropriate process design for COD reduction of the dairy manufacturing waste stream, based on an established limits and operating conditions, and (3) to confirm the selection of the appropriate approach based on the fouling rates of the membrane filtration system.
4. Conclusions
The application of membrane filtration to the management of waste water streams from a dairy manufacturing operation has been completed. The waste water stream had a pH of 11.32 ± 0.55 and was heterogeneous in terms of COD, protein and total solids. A Dissolved Air Flotation (DAF) system removed large particles, total solids and proteins. The DAF system was not effective in reducing the COD of the waste stream to below a target COD of 1200 ppm on a consistent basis.
All six membranes (200, 20, 8, 4, 0.083, 0.058 kDa) reduced the permeate COD to less than 1200 ppm. These reductions were independent of the COD of the feed stream. The permeate COD from the membranes was not influenced significantly by pressure. The reduction in permeate COD improved as the membrane pore size decreased.
Two membranes with operating pressures of 100 psi (200 and 20 kDA) and two with operating pressures of 350 psi (8 and 4kDa) were selected for extended fouling studies. The permeate flux from the four membranes decreased with time due to fouling, and the decrease in flux was described by a three-parameter expression, including a fouling rate constant. The 20 kDa pore size membrane had the lowest fouling rate in the extended fouling study compared to the other three membranes. In addition, the 20 kDa pore size membrane operated at a lower pressure than the 8 or 4 kDa pore size membranes.