Field Evaluation of Membrane Separations in Rendering Facility Wastewater Treatment

Date: November 21, 2020

Principal Investigators: David A. Ladner, Clemson University

Keywords: Membrane, Wastewater Treatment

Summary:

This project was an effort to decrease costs for wastewater treatment at rendering plants. One of the most widely used processes in the rendering wastewater treatment train is dissolved air floatation (DAF). DAF requires a large dose of various chemicals and significant operator time to make it work well. We are investigating whether membrane filtration may be a cheaper alternative.

 

Membrane filtration has been gaining ground in many industries. Membranes are used for juice production, milk separations, and pharmaceutical processes. Lately, there has been a movement toward using membranes to replace some of the distillation columns in petroleum refineries. And a key place where membranes are gaining ground is in environmental applications like drinking water and wastewater treatment. It is quite possible that the municipal wastewater treatment plant that receives your home’s wastewater has already converted to membranes for part of their treatment train.

 

There exists a wide variety of membrane types. They are classified first by pore size. The membranes of interest for this project have a pore size of about 0.1 to 0.2 μm. By comparison, bacteria are typically about 2 μm in size, so these membranes can even remove bacteria from the wastewater stream. Membrane material is another important classification. Polymeric (plastic) membranes are most common, but ceramic membranes are gaining ground, especially where the water is highly acidic, hot, or corrosive.

 

For this project tested whether membranes could replace DAF. The first reason membranes might be better is because they may not require as much chemical input as DAF. With DAF, operators need to add acids or bases to get the pH of the water to the correct level. Then they need to add more chemicals, called flocculants, to help fats and proteins aggregate so they can be floated to the surface with air bubbles. If the chemical inputs aren’t correct, DAF does not work well. On top of this, another downside is that the chemical input interferes with the fats and proteins that the renderers want to recover. The flocculants are often a contaminant that must be monitored closely so the fats and proteins are still usable. Lastly, the dissolved oxygen in DAF causes a breakdown in the fats and proteins, meaning DAF is breaking down the very product that renderers sell to customers.

 

With membrane processes, chemicals are not needed during filtration. The only time one needs to add chemicals is during the membrane cleaning cycle. Because it is only during this short cleaning cycle, and because the chemicals are not diluted by the full wastewater flow, the chemical inputs are vastly reduced. Our experiments suggest that the only thing needed to clean the membrane is sodium hydroxide. That’s something that the wastewater probably needs, anyway, before it goes to the next step in the treatment train. The amount of sodium hydroxide needed for the membrane is small compared to what DAF would require.

 

Our work tested whether the membranes could be operated with real wastewater for long time periods. A key to making it work is in the automation; the membranes need to be backwashed and cleaned periodically. We do not want to occupy an operator’s time to do that, so the automated system was optimized to perform the functions. The results are promising; the membrane system treated wastewater effectively and the groundwork is laid for taking the next steps to put this process in practice in the industry.