An introduction to wastewater
Food manufacturing inevitably results in the generation of large volumes of mostly biodegradable liquid and solid waste. Industrial wastewater streams have to be pretreated on site before a discharge to municipal treatment plant, or fully treated ahead of discharge to streams, rivers, lakes or oceans.
Different sources contribute to the generation of wastewater in food industries, including meat processing, dairy products, seafood and fish processing, fruits and vegetable processing, starch and gluten products, confectionery, sugar processing, alcoholic and nonalcoholic beverages, rice and bean products among others.
Wastewaters released from these industries are turbid, with high concentrations of total suspended solids (TSS), chemical oxygen demands (COD), biological oxygen demand (BOD), fats oils and grease (FOG), and usually nutrients such as nitrogen (including ammonia) and phosphate. Fortunately, content of hazardous chemicals is low. Other characteristics of food processing wastewater are a) large seasonal variation, b) large hourly variation in concentration and type of contaminants, c) often small and variable flows per day, d) colored effluent, and e) unbalanced concentrations of carbon, nitrogen and phosphorus, which complicates bioreactor treatment.
High concentration wastewater (suspended solids over 10 percent by mass) can be directly dewatered and used or recycled in rendering plants. Low concentration wastewater, such as discharge from boilers or cooling towers, can be discharged without any treatment in most countries.
New developments
No matter where food manufacturing wastewater is discharged, new regulations encourage primary treatment to reduce amount of TSS, FOG and BOD/COD in the effluent. This step helps with the odor problem and significantly reduces potential fees and fines from the regulators agencies.
Food processing wastewater presents many challenges to the classical primary treatment technologies and flotation systems. Such wastewater contains very high amounts of contaminants, up to 500 times higher than in the municipal or low strength industrial wastewater influents. Depending on what is processed the wastewater influent can change hourly, daily or weekly. The space available for the wastewater plant is often very limited. Wastewater treatment produces large amount of sludge with low solids content that have to be dewatered before recycling of FOG or proteins are possible. The cost of coagulants and flocculants needed for primary treatment can also be very high.
It would be advantageous to design and develop a food processing wastewater primary treatment system that would answer to these challenges. A system with a small footprint, that can respond fast to changing wastewater strength, produce sludge with high solids loading and help eliminate the cost of treatment chemicals is necessary.
Academic and industrial communities did respond to these challenges. This resulted in the development of new primary and secondary treatment systems.
New primary treatment technologies
Sedimentation is one of the favorite gravity-separation methods to remove contaminants in water treatment. Most fats oils and grease have low density and cannot be separated by sedimentation from water streams. Thus, flotation is a much more suitable technique to remove oil and particles with low density from water during or after de-emulsification.
One of the key steps in the flotation method is the introduction of air bubbles into water. In early flotation machines, coarse bubbles (2 to 5 mm) were introduced into the contaminated water by blowing air through canvas or other porous material. In some impeller-based machines, air could be introduced from the atmosphere without compressors or blowers. This type of flotation, in which impeller action is used to provide bubbles, is known as induced-air flotation (IAF) and also produces fairly coarse bubbles.
Another flotation method, called dissolved-air flotation (DAF), is common in the treatment of oily wastewater. In DAF, a stream of wastewater is saturated with air at elevated pressures up to 5 atm (40-70 psig). Bubbles are formed by a reduction in pressure as the pre-saturated water is forced to flow through needle valves or specific orifices. Small bubbles are formed, and continuously flowing particles are brought into contact with bubbles. Such bubbles rise very slowly to the surface of the tank. This is the main driver of the large dimensions for DAF tanks.
Air solubility also limits the amount of gas and gas bubbles available. Furthermore, to avoid clogging of orifices, only a small fraction of pretreated water is aerated and then recycled into the tank where bubbles nucleate under already preformed flocs. Therefore, the number of bubbles is limited and treatment of high strength industrial wastewater with high TSS and FOG loads is very inefficient.
To answer these problems, centrifugal, jet and cavitation flotation systems have been developed. In these systems centrifugal forces have been used to produce smaller bubbles, which were created mechanically. Centrifugal flotation systems are based on liquid hydrocyclone technology. Contact of air, contaminants and chemicals occurs inside the hydrocylone column under the influence of centrifugal forces. Solid-liquid separation occurs inside the column. This results in much faster response flotation units with smaller footprint. Flotation tanks are only used for sludge skimming. However, larger bubbles cannot remove small particles and dissolved air flotation still produces wastewater with much better contaminant removal efficiencies.
Moving bed biofilm reactors
Secondary treatment is widely used in industrial wastewater treatment for the removal of dissolved organic biodegradable materials (BOD’s). Such technologies were developed for municipal wastewater treatment where concentration of contaminants (TSS, BOD, FOG, nutrients) rarely fluctuates and is almost constant. However, in industrial waste streams the concentration of contaminants often changes hourly. The amount of contaminants can be two orders of magnitude higher than in municipal wastewater. This presents a serious challenge for the classical activated sludge and sequence batch reactors.
Biofilm bioreactor processes are increasingly being favored instead of activated sludge or SBR processes. There are several reasons for that: smaller footprint, less sludge produced, no return activated sludge needed, biosolids that are easier to separate are produced and attached biomass is more specialized (higher concentration of relevant organisms) at a given point in the process train.
Moving bed biofilm reactors (MBBR) are a hybrid of activated sludge and biofilter processes. Contrary to most fixed film bioreactors, MBBR utilize whole tank volume for biomass. However, contrary to activated sludge reactor, MBBR does not need return activated sludge (RAS).
This is achieved by having a biomass grow on plastic high surface area carriers that move freely in the water volume of the reactor, kept within the reactor volume by a sieve arrangement at the reactor outlet. At the bottom of the tank, large bubble aeration system assures mixing and floating of plastic carriers with attached biomass. If anaerobic or anoxic reactor is used, a large banana type mixer is attached to the tank wall for efficient mixing of the media and wastewater.
The biofilm carrier is made of high density polyethylene (density 0.95 g/cm3) and shaped as a small cylinder with a cross on the inside of the cylinder and “fins” on the outside. The original cylinders have a length of 7 mm and a diameter of 10 mm. Later, various shapes and sizes were introduced by numerous manufacturers. One of the important advantages of the moving bed biofilm reactor is that the filling fraction of carrier in the reactor may be subject to preferences. That means that by increasing the filling fraction one can increase the capacity of the bioreactor to reduce BOD’s without additional tanks.
In short, the advantages of MBBR systems when compared to activated sludge and SBR processes are as follows: more compact design; easy to expand, single pass process; less sludge produced; easy to separate biosolids; easy and fast response to change in load of contaminants; process resiliency (much less sensitive to toxic shocks); minimal maintenance; and one tank can be used for both COD/BOD removal and nitrification/denitrification.
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