Wastewater sludge reduction in industrial settings using oxidation by ozone

Biosolids management is among the most important challenges of the wastewater industry due to its high cost and limited options of environmentally safe disposal methods. On average, biosolids management accounts for approximately half of the total costs of a typical wastewater treatment plant. Therefore, minimizing biosolids with low-cost sludge reduction solutions is of key interest to the industry. Ozone is one such solution.

What is ozonation?

Ozonation is a highly effective sludge reduction technology in wastewater treatment plants. It’s a chemical treatment technique based on the injection of ozone into biological sludge, the untreated precursor to biosolids. Ozone (O3) is a powerful oxidant with a fast reaction time, no added chemicals and reduced disinfection by-products. It’s an unstable compound, a reactive oxygen species, that degrades organic and inorganic substances in wastewater. This treatment process of wastewater is considered a compact and clean technology.

How is ozonation used to treat sludge?

Ozone is a biological sludge reduction technology that works by oxidizing the biological sludge and ultimately reducing the overall amount of sludge that is produced during wastewater treatment. Reducing sludge decreases operational costs and energy expenditure to produce treated biosolids. Ozone treatment of sludge makes the waste treatment process less expensive and more sustainable.

How does ozone injection achieve sludge reduction?

The use of ozone in biological sludge treatment can result in a 40% decrease in sludge volume. This is accomplished through the following mechanisms:

1. 1. By perforating and rupturing their cell walls, ozone inactivates the microorganisms in the biological sludge. With their cell walls breached, the cells lose their cytoplasm, which increases the amount of organic carbon dissolved in the sludge.
2. 2. Weak, filamentous bacteria are destroyed through complete cell lysis.
3. 3. Strong, beneficial nitrifying bacteria, like Protozoa, resist and are only partially damaged. But instead of replicating, they metabolize the energy and carbon released from the weaker bacteria to repair themselves, further reducing sludge mass.
4. 4. The mineralization or conversion of inert solids into more biodegradable forms further contributes to the overall decrease of sludge mass.
5. 5. When ozonated sludge, which has a high concentration of soluble chemical oxygen demand (SCOD), is recirculated to the aeration tank, cryptic growth occurs. Microorganisms in the aeration tank utilize the extra oxygen as a secondary external fuel supply for growth and cell maintenance. This process converts ozone-solubilized sludge into respiration products (CO2 and N2), achieving a net reduction of sludge.

What are the benefits of using ozone to treat biological sludge?

1. 1. Reduced overall biosolids production.
2. 2. Reduced operating costs. With a lower volume of biosolids, biosolids disposal costs are reduced. The treatment cost of biosolids also is reduced because less chemicals, equipment and energy are required.
3. 3. Reduced energy usage. The biological secondary treatment requires less energy because less air is needed from blowers due to oxygen recovery, which supercharges the activated sludge treatment process.
4. 4. Better removal of pathogens and volatile solids.
5. 5. Enhanced biodegradation of emerging contaminants, such as 1,4-Dioxane, pharmaceutical micropollutants and personal care products.
6. 6. Strong release and recovery of nutrients.
7. 7. Reduced foaming and better removal of filamentous bacteria due to the fresh oxygen being added to the aeration basin.
8. 8. Better dewatering. Due to the reduction in filamentous bacteria, less ECP (extra cellular protein) is present, so there is less water in the flocs and better dewatering is achieved. ECP, which has a negative effect on dewatering, is oxidized by ozone. ECP is produced mainly by filamentous bacteria and therefore reducing this family of bacteria has a positive impact on foaming, dewatering and sedimentation. Beneficial floc structures stay intact and stronger due to less ECP in the effluent.
9. 9. Reduced odors. Ozone treatment can help neutralize odorous compounds by oxidizing them, leading to a reduction in odor emissions from the sludge.

Case study: Lariana Depur Wastewater Treatment Plant

In the heart of the Italian textile industry, the Lariana Depur water resource recovery facility in Como treats residential and industrial wastewater – which includes multiple textile manufacturers. In 2018, two Capital Controls® DTPF 182XTL-O ozone generators were installed to replace older generation ozone equipment and optimize the plant for lower specific energy consumption and less expensive spare parts. Included in the system was a liquid oxygen feed system, ozone destruct units, power-supply units and a redesigned porous diffuser system to optimize ozone transfer efficiency.

Each ozone generator is designed for a maximum capacity of 600 kilograms (1,322 pounds) per day. Ozone production can reach up to 36.4 kg per hour (1,925 pounds per day), with cooling water demand of 32.7 cubic meters per hour (144 gallons per minute).

In addition to removing color from the wastewater, the Lariana Depur system addresses additional treatment needs including chemical oxygen demand (COD) for sludge reduction, surfactants and disinfection before discharge to the Como River — all at significantly lower energy usage. Through ozone injection in a side-stream of the return sludge line from the secondary clarifier to the aeration tank, Lariana Depur reduces 25 percent of their excess sludge, saving significant disposal costs and improving the sludge volume index (SVI) by 30 percent. Lower SVI means better sedimentation and less chemical consumption. In the disinfection process, filamentous bacteria –the main cause of foaming and bulking that have negative effects on sludge sedimentation – are destroyed.

Other De Nora ozone installations have experienced sludge reductions of 40% in the food industry, and from between 30% and 53% in other installations. While the greatest advantage of biological sludge oxidation is that it can significantly reduce biosolids production while also lowering operating and energy costs, additional benefits include reduction in lime slurry by 20%, iron chloride by 37%, dewatering costs by 30%, incineration costs by 40%, filter costs by 25%, transport costs by 25% and filamentous bacteria by 90%.

Why work with De Nora?

As an original pioneer of ozone and advanced oxidation treatment solutions, De Nora has developed and delivered ozone generators for water treatment and advanced oxidation processes (AOP) for industrial applications since the 1970s, supporting customers from design to commissioning and aftersales. Systems range in size from 20 PPD (500g/h) up to 6,000 lb/day (113 kg/h) and offer flexibility, reliability, and cost advantages.