Reducing sludge and biosolids is vital in the wastewater treatment process. Aside from reducing sludge, wastewater treatment facilities need to treat and dispose of the excess sludge and byproducts produced. While the activated sludge process is a common method for the biological oxidation of wastewater in both municipal and industrial facilities, it poses significant operating costs. Handling of excess sludge can encompass the following costs:
- Chemical additions such as polymers for dewatering and enhanced settling
- Sludge handling on site
- Electricity
- Transportation costs
- Disposal costs
In addition, in some parts of the world, it is becoming more difficult to dispose of the excess sludge at landfills/farmland or by incineration. While the cost of sludge disposal is rising everywhere, in regions with restrictive policies, the cost and logistics of disposing of the sludge are becoming problematic.
Ozone has been shown in the literature and in commercial applications to offer an alternative method for reducing excess sludge. This process involves exposing a portion of the activated sludge to ozone. Some of the bacteria that make up the sludge are lysed via exposure to ozone, releasing COD into the wastewater. Even though some of the cells are damaged or killed, the overall process generally does not show an increase in BOD/COD leaving the process.
Depending on the amount of ozone required per unit of sludge removed and the costs associated with the current handling of the excess sludge, ozone can represent an economical alternative to conventional sludge handling processes. Ozone also offers other potential advantages to the activated sludge process, including a reduction in bulking/foaming, scum control, and improved denitrification.
The ozone/sludge contacting method appears to impact the ratio of the amount of ozone applied to the amount of sludge removed. As with most ozone applications, efficient production and application of ozone is critical to process economics. In the case of sludge reduction, ensuring that the ozone is used to lyse the cells versus react with the expelled COD is a key factor in the overall efficiency of the ozone-based process. A table from a previous Ozone News article (Raugust et al, 2009) and modified to include additional data shows the range of ozone utilization rates from various investigators.
Ozone Consumption per Unit of Sludge Removed
Table 1
| Reference | Ozone Consumption kg O3/Kg TS Reduced |
| Sakai et al. (1997) | 0.165 |
| Sakai et al (1997) | 0.133-0.178 |
| Kobayashi et al. (2001) | 0.25 |
| Sievers et al. (2004) | 0.395 |
| Raugust et al. (2009) | 0.07 |
| Capital Control Ozone (2017) * | 0.11 |
Below, Tables 2 and 3 show information on the Capital Controls Ozone sludge reduction process.
Commercial Applications of Ozone Sludge Reduction
Table 2 shows several commercial-scale facilities that adopted ozone and the associated reduction in excess sludge.
Table 2
| Facility | Type of Facility | Sludge Reduction |
| Hercules Doel BVBA | Chemical | 45% |
| Alfra Mockrehna | Chicken Processing | 67% |
| Tento 1 | Municipal | 35% |
| Coelsanus | Food | 40% |
| Trento 2 | Municipal | 37% |
| Verese | Municipal | 36% |
The Capital Controls Ozone approach to efficient ozone sludge contacting is illustrated below in a simplified process flow sheet:
Table 3 shows calculations for an upcoming installation in the US, illustrating the breakdown of savings by type and the associated costs.
Case Study: Cost Savings Using Ozone for Sludge Reduction
These data in Table 3 have been provided by DeNora Water Technologies for a U.S. chemical plant (proprietary client). The system for this application would produce 6.4 kg/hour of ozone.
Table 3
| Annual Sludge Costs | |
| Transport | $ 83,258 |
| Landfill | $ 44,374 |
| Polymer | $ 117,260 |
| Roll Off Boxes | $ 10,710 |
| Electricity | $ 7,934 |
| Total Direct Costs | $ 263,537 |
| Ozone Costs | |
| CAPEX Annualized | $ 81,600 |
| Electricity | $ 65,000 |
| Total Ozone Costs | $ 146,600 |
| Savings at 35% Sludge Reduction Using Ozone | $ 116,937 |
As data show, certain locations operating activated sludge plants can benefit economically from ozone-based sludge reduction. A challenge in developing these projects is the difficulty in piloting the process to demonstrate both the sludge reduction and maintenance of the plant’s COD removal efficiency, given the complexity of combining the ozone pilot with an activated sludge pilot.
Acknowledgement:
I would like to thank Alex Bettinardi and DeNora Water Technologies for some of the data presented in this article and their assistance in reviewing the article.
References:
Kobayashi, T., Arakawa, K., Katu, Y., & Tanaka, T. (2001). Study on sludge reduction and other factors by use of an ozonation process in activated sludge treatment. In Proceedings of 15th Ozone World Congress, London.
G. Raugust, L. Liberati, R. Novak, & P. Wrampe (2009). Full-Scale Application of a Novel Sludge Ozonation Process for Achieving 40% and up to 80% Excess sludge Reduction at a 25,000m3/day Municipal Wastewater Plant. Ozone News Vol. 37, No. 3
Sakai et al. (1997). An activated sludge process without excess sludge production. Wat. Sci. Tech. 36 (11) 163-170
Sievers et al. (2004). Sludge treatment by ozonation – Evaluation of full-scale results. Water Science & Technology Vol 49 No 4 pp 247 – 253
Yasui et al. (1996). A full-scale operation of a novel activated sludge process without excess sludge production. Wat. Sci. Tech. 34 (3-4) 395-404