As an indicator of the efficiency of sludge reduction process, sludge disintegration number (SDN) was introduced. It was defined as the ratio of the amount of treated to the amount reduced (Yoon, 2003). Based on the initial studies performed with ozone, SDN was approximately 3 when ozone dose was 0.05 g O3/g MLSS each time mixed liquor was treated (Yasui, 1996; Sakai, 1997). As a result, the specific ozone demand to reduce sludge was around 0.15 g O3/g MLSS removed. In the same study performed in an activated sludge plant with a clarifier, no excess biosolids was removed from the system for almost a year. This observation suggested that ozone converted non-biodegradable matters to biodegradable so that non-biodegradable matters did not accumulate indefinitely.
There are two major factors affecting the economics of biosolids reduction.
- Cost of sludge disintegration – The capital and operating costs of sludge disintegration process vary widely depending on the method of sludge disintegration. Biosolids reduction using biological methods such as anoxic, anaerobic, or themophilic reactors are less costly than chemical or physical methods, but according to some reports the efficacy is fuzzier than chemical and physical methods.
- Extent of target biosolids reduction – Cost of biosolids reduction increases disproportionally when the target biosolids reduction increases. For example, reducing 30-50% biosolids is much more economical than reducing 70-90%. It is because sludge disintegration process becomes less efficient when complete disintegration (or solubilization) is targeted. In addition, the disintegrated sludge sent to aeration tank can come back to the disintegration process before biologically treated.
The following calculator is for the cost of ozonation to reduce sludge. As remarked in the calculator, the biggest uncertainty is in the cost of additional aeration to treat additional COD load from the disintegrated sludge. Depending on the amount of COD oxidized directly by ozone, the amount of oxygen dissolved during the ozonation process, the amount of oxygen recovered in denitrification process, etc., the cost of additional aeration can vary quite significantly. The total operating cost is likely in between the costs with and without additional aeration. One must note the cost calculated here is only for operation. Capital costs for ozone generator and ozone contactor are a significant portion of ozone application in general.
© Seong Hoon Yoon