Wastewater - Ozone Treatment
Wastewater Treatment Processes (WWTP) are used to treat municipal as well as industrial wastewater to meet effluent standards prior to discharge in the natural environment. Wastewater discharges from industrial sources contain a wide range in levels of COD, BOD, TSS and other emerging contaminants (i.e. pharmaceuticals, aldehydes, glycol, amines, alcohols, complex proteins, etc). This includes process water from industries such as textile plants, dairies, breweries, pharmaceutical industry, paper & pulp, cosmetic, chemical and other production facilities.
The major contaminants found in wastewater are biodegradable organic compounds, volatile organic compounds, xenobiotics, metal ions, suspended solids, nutrients such as phosphorous, nitrogen, microbial pathogens and parasites. The approximate composition of organic carbon found in typical sewage wastewater are carbohydrates (11-18 %), proteins (8-10 %), free amino acids (0.5-1.5 %) fatty acids (23-25 %), dissolved organic acids (7-11 %) and other organic compounds (25-28 %).
A top view of a complete wastewater treatment solution can be found below:
Sedimentation is a primary step which involves flocculants such as iron salts, aluminum, polyelectrolytes and lime for precipitation to settle solids from water by gravity. The effluent from primary treatment will typically contain considerable organic material and will have a relatively high biological oxygen demand (BOD). The average particle size range found in wastewater are found below:
Around 50% of the raw wastewater COD and BOD can be removed as a part of the settled solids. Settled sludge is typically treated further in the plant’s sludge handling steps.
Ozone has a complex impact on water/wastewater parameters – it reduces color, improves taste, odor, kills bacteria, viruses, oxidize iron, manganese, cyanide, phenol, benzene, chlorophenol, atrazine, nitrobenzene and other pollutants. Ozone application shall increase the biodegradability of wastewater and the COD:BOD ratio from an industry water is shown as an example below:
|Parameters||Raw water before ozonation||Raw water after ozonation|
As seen in the table above the ratio of COD/BOD and biodegradability was increased 10 times due to ozonation. This is one of the potential application in treatment of complex industrial wastewaters prior to the biological process.
This is known as biological or activated sludge process and involves the use of microorganisms to breakdown organic pollutants, total nitrogen content and phosphorus. This process involves aerobic, anoxic and anaerobic treatment through various technologies and design such as Sequential Batch Reactors (SBR), conventional treatment or Membrane Bioreactors (MBR). Aerobic treatment utilizes aerobic heterotrophic bacteria to break down BOD and ammonia-N and organic-N. This produces nitrate which can then be converted into nitrogen gas through anoxic treatment:
NH4+ +O2 → NO2- + 2H+ +H2O
NO2- + O2 → NO3-
|Denitrification||NO3- → NO2- → NO → N2O → N2|
Anoxic treatment, as opposed to aerobic treatment, is characterized by an oxygen free environment, where bacteria must use oxidized nitrate for respiration. Ozone can play an important part in avoiding undesirable filamentous bacteria in the biological process. See below for more information.
Anaerobic treatment can be incorporated upstream from the aerobic and anoxic zones in order to achieve a net reduction in phosphorus.
Most of the readily biodegradable material is removed during the biological process and non-biodegradable material remains in the effluent. The net growth of biomass in the biological treatment process is removed through secondary sedimentation as Waste Activated Sludge (WAS). The remaining settled sludge is recirculated.
Ozone Treatment of Return Activated Sludge (RAS)
MLR is an internal recirculation from aerobe to anox tanks and RAS is from secondary sedimentation to incoming flow from primary treatment.
The high water content, compressible and colloidal nature of the sludge are common characteristics found in WWTP. Filamentous bacteria are a normal part of the activated sludge microflora while excessive long filaments lead to sludge bulking and prevent flocculation. Using ozone in the RAS flows promotes the growth of floc forming bacteria and inhibit the activity of filamentous bacteria which enhanced sludge bulking and sedimentation. Low concentration of dosage of ozone can be used as an approach to promoting floc formation and inhibit the activity of filamentous bacteria and sludge bulking during process.
Industrial Waste effluent (BOD/COD) ozone treatment
Common sludge thickening process by physical means are chemical settling, gravity settling, floatation, centrifugation, gravity belt and rotary drum. The sludge thickening process increases the portion of the liquid fraction from the sludge. These liquids have high COD and BOD values depending on the raw materials used in process industries. Typically COD range for chemical process wastewaters are between 400 – 40 000 mg/L and domestic wastewaters in the range of 100 – 450 mg/L. Generally COD above 350 mg/L range requires additional treatment and more information can be found in section 6 below. Please click here to read more about BOD and COD treatment with ozone.
Ozone treatment of digested sludge
Anaerobic digestion is a sequence of biological processes in which microorganism breakdown biodegradable material in the absence of oxygen. During anaerobic treatment about 50 % of the organic matter in the sludge is susceptible to biodegradation into biogas, the other half of the organic material being more recalcitrant and degrading slowly. Sludge ozonation leads disintegration of complex organic substances into bio-degradable low-weight molecular compounds, which increases the biogas yield.
Ozonation of thickened sludge permeate
Digested sludge typically contains 2-3 % total solids. This sludge requires thickening by mechanical means through a centrifuge or screw press, which leaves around 20 % dry solids. This process leaves highly concentrated waste liquid which is suitable to treat by ozonation if it cannot be reintroduced to primary treatment. For example food and beverage manufacturing facilities typically generate high strength waste streams as a by-product from their manufacturing operations.
Ozone treatment of biogas odor
Biogas facilities often use manure and meat scraps to produce biogas. Biogas typically refers to a mixture of different gases produced by the breakdown of organic compounds and formation of Sulphur-based compounds (for example H2S and mercaptanes) that have very bad odor and are toxic to health). As a solution, ozone breaks down these compounds, which significantly reduces odors with no harmful by-products. This leads to improved air quality in the surroundings as well as improved work environments. More information can be found here.
The purpose of tertiary or advanced treatment is to provide a final step to raise the effluent quality or as a disinfection polishing step.
Tertiary treatment may not be needed to all wastewater treatment plants and it varies from one plant to another, depending on the type of water contamination. Some common advanced WWTP include removal of nutrients, non-biodegradable organic matter, suspended solids and toxic materials. Among other technologies commonly used methods are filtration, MBR, RO, UV and AOP. These will be covered in more detail in the following sections.
The most common granular media used in filtrations are sand and activated carbon. The important factors for pollutant removal include the amount of media in the column and the contact time.
|Simple to setup||Frequent clogging/filter changed|
|Broad range in pore size or surface area||Reduces the contact on the surface due to accumulation of bacteria over time|
Ozone can be used as a pre or post treatment step to filteration that break down the particles and improve the quality of water.
Reverse Osmosis (RO) ozone permeate
RO is often used in commercial and residential water filtration. The water is moved across the membrane against the concentration gradient, from lower to higher concentration.
|Removal of both good and bad ions||Frequent clogging and chlorine concentration damages the RO system|
|Determines the taste of water||Efficiency reduces over period of time|
Ozone is an optimal method to disinfect or treat further the RO permeate to reclaim the water for other purposes or to comply with very high effluent regulations.
Disinfection of water using ozone is advantageous compared to more traditional methods, such as chlorine or UV disinfection. Ozone effectively breaks down the lipid layers in the cell membrane. Firstly, ozone is more effective at deactivating viruses and bacteria than any other disinfection treatment, while at the same time requiring very little contact time, more information can be found here.
Municipal wastewater treatment with ozone
The below graph is a typical example from treated wastewater using ozone as a one-step technology in reduction of COD and BOD levels at our pilot system.
More information about feasibility studies and pilot project can be found here.