We have decided to relaunch our blog with an emphasis on the types of questions we receive from customers. We have observed that companies and government entities that contact us have heard that ozone or advanced oxidation might be an option for treating their air or water challenges, but don’t have a good feel for what is involved in the application of these technologies. As a result, we will try to make provide some explanations on how these technologies work and what is involved in a commercial application of the technologies.
One of the questions we receive consistently is the ability of ozone to treat BOD (biochemical oxygen demand). Limits on BOD are often imposed on facilities to that discharge their wastewater either to surface water bodies or to publicly owned treatment works (POTW). In the former case, the concern is that the wastewater will cause a reduction of oxygen in teh surface water body which can have an adverse effect on fish and other aquatic species.
In the case of the POTW, which may be designed to handle BOD in general, high BOD loads from customers on their collection system adds to the facilities cost. As a result, many POTW add a surcharge to the cost of handling high levels of BOD. Surcharges range from a few cents per pound of BOD discharged to $0.60 per pound of BOD discharged. Normally these charges begin to accrue after the level of BOD exceeds 250 ppm. In some cases, the facility is not allowed to discharge more than a certain amount of BOD, forcing the facility to provide its own treatment system, sometime referred to as pretreatment since the final treatment will take place at the POTW.
Chemical oxidation systems such as ozone and other advanced oxidation might be attractive options for treating BOD since they are relatively simple to operate versus biological systems and are more compact than biological systems. There are a number of considerations, however, that need to be taken into account before selecting such an option.
Mixing some ozone with the wastewater seems like a simple way to solve a BOD problem. Essentially, all you would need is an ozone generator and a way to mix the water and ozone together. The ozone is made on site, requires no storage, breaks down to oxygen when the reaction is completed and does not form toxic byproducts.
In some cases, ozone might be a solution for reducing BOD on this basis, but in actual applications we have found that it is more complicated. First, ozone tends to break down molecules to smaller more biodegradable molecules. In some situations, it is making BOD. Certain compounds can’t be broken down further by ozone such as small chain ketones, aldehydes and carboxylic acids. Some reduction of BOD is observed, but the reductions are more on the order of 30%.
If BOD reduction is required and biological systems are not an option, consideration should be given to advanced oxidation process (AOP) such as ozone peroxide or UV/ozone. The processes result in the production of a compound known as the hyrdoxyl radical. Unlike ozone, this compound can breakdown most organic molecules to carbon dioxide and associated ions. the reactions are fast and non selective so it can work against a wide range of compounds. The disadvantage of AOP is the processes can be expensive. This means it might be cheaper to pay the surcharges or invest in a biological treatment system.
AOP are best used when the compounds can’t be discharged to the POTW or surface water without treatment and biological processes can not break down the compounds. This might be the case for toxic biorefractory compounds.
In some cases, the fact that ozone breaks down compounds into a biodegradable form may be an advantage of the ozone is match with a biological process. The ozone can be used to take toxic or slow to digest compounds and make them more easily handled by the biological system. In addition, the ozone adds oxygen to the system which enhances the operation of aerobic biological systems. In textile wastewater treatment, improvements of 20-30% have been observed with pretreatment with ozone.
So, in considering the use of chemical oxidation, it is important to look at the nature of the compounds that make up the BOD, the economics of biological system, space availability, POTW surcharges and the cost of the chemical oxidation system to make a proper selection. It is also likely that pilot testing will be needed to develop the engineering data to design a system. Chemical oxidation systems don’t make sense in every application where BOD is involved, but depending on the situation they might make a good solution. Factors that would favor chemical oxidation would be:
- Compounds with Long Digestion times
- Relatively Small Flow Rates
- Limited Space for Treatment Equipment
- Variable Levels of BOD Loading
- Applications Where COD (Chemical Oxidation Demand) is more important
- Applications Where a Biological System is in place and can be Enhanced by Ozone
Spartan welcomes inquiries regarding applications where BOD, COD, TOC (total organic carbon) reductions are required. When chemical oxidation makes sense we will do our best to deliver a solution, if chemical oxidation is not an option, we will let you know that as well.