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SSI MBBR Problem Solver – Dairy Dangers

By: Doreen Tresca
Post Date: August 5th 2020

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Dairy wastewaters are notoriously difficult to treat. At a close look, the truth is really in the chemistry. Every dairy application is different. No two plants use the exact same combination of formula, equipment, inlet water quality, etc. As water and wastewater professionals, we should always strive to understand and treat the conditions of one plant independently of another.

 

When we take time to consider the entire supply chain, from feed stock to type of animal, you will find that the outcome can be highly variable from one plant to the next. A farmer may change feed or food source during spring to summer, or climate conditions impact the animal’s caloric intake. Many studies have shown the level of urea in milk is highly variable based on protein intake. Ultimately, it all ends up in the wastewater treatment plant. This principle not only applies to dairy, but also to a number of industries (textile, food and beverage, leachate, etc).

 

My team and I are frequently asked to consult on plants with decreasing efficiency, diagnose ongoing operational challenges, evaluate performance enhancements, etc. Recently we were contacted by a yogurt manufacturer to diagnose declining WWTP efficiency. We evaluated several existing processes at the plant including mechanical and biological systems, but also the specific chemistry of the wastewater. Upon close examination, a significant amount of organic nitrogen, cellulose and calcium salts were found. Raw influent COD values often exceeded 10,000 ppm, with average COD values between 4,000 and 5,000 ppm. Total nitrogen values often exceed 150 ppm. While the raw influent ammonia values were often below 2 ppm, we found the Ammonia-Nitrogen in the plant increasing from process to process. Our assessment was that a combination of chemical precipitation and unaccounted organic nitrogen breakdown had led to declining treatment efficiency. The existing biological system had failed to take in account high levels of calcium salts, cellulose and the propensity of these chemicals to precipitate in solution.

 

Due to the clients limited time for process disruption, as well as concerns over infrastructure modifications, we felt that a properly designed MBBR system would help the end user increase efficiency with shortest downtime and best return on investment. For anyone who has designed, owned or operated an MBBR, it is no secret they can be a powerful treatment method.

 

MBBR systems are typically designed using a surface area loading and surface removal relationship (i.e. Grams of substance per protected surface area of biofilm carrier). In general, SALRs follow a semi natural log approach with temperature being a primary variable. In other words, the higher the temperature, the higher the SALR can be achieved while maintaining 90% SARR relationship. Most of our designs are based on 90% removal efficiency. Note, there is no “one” curve or graph for MBBR systems. Every wastewater source has its own unique design approach due to a combination of factors. In many areas, I’ve witnessed designers simply using published values without understanding water chemistry and the resulting interaction of biofilm carriers. Some are lucky, while others fail miserably. The design and application of MBBR should always take into consideration the following:

 

 

 

 

 

 

 

 

 

We ultimately provided design and supply of an enhanced MBBR process, utilizing full floor cover diffused aeration and virgin HDPE biofilm carriers with 589 m2/m3 of effective surface area. Higher SA carriers were considered, but due to the inorganics present and high COD loading, a lower SA carrier was chosen to ensure sufficient diffusion-able area was available for best long term operation. Considerable time was given to the best aeration design, with both Fine Bubble and Coarse Bubble options discussed. While the fine bubble provided the best energy balance, stainless wide band coarse bubble aeration was chosen to reduce maintenance. The MBBR was chosen due to its operational simplicity, ease of upgrade and high rate of treatment. Upon commissioning, the plant was immediately able to achieve higher degree of organic reduction while simultaneously reducing the power draw of the existing blower system. Despite using coarse aeration, the increase mass transfer (alpha value) and oxygen transfer allowed the plant to reuse the existing blower network. The increased biological efficiency ultimately allowed the end user to easily process another 20% flow rate within the existing infrastructure while maintaining the same effluent COD values. The MBBR supplied operates as a 2-stage system, with each stage operating at ~33% by volume.