Water scarcity is one of the most pervasive challenges facing communities globally and the Middle East and North Africa is the most water scarce region in the world. With approxima...
Critical first step
One proven approach to help meet growing water demands, while safeguarding existing water supplies is water reuse. Biological wastewater treatment - a crucial first stage in the treatment of wastewater for reuse - is a process which removes more than 90 percent of the pollutant load, undertaken before polishing treatment and disinfection to meet reuse standards. Using naturally occurring elements, biological treatment is a safe and sustainable method of removing the pollutants from wastewater. Water reuse technologies produce high-quality water at a lower life-cycle cost than developing a new water supply, and deliver a resilient water source with valuable economic and environmental benefits.
The most common form of biological treatment is known as activated sludge whereby air is introduced at the bottom of a tank as an oxygen source. A mixture of untreated wastewater (food and return-activated sludge containing live biomass enters the tank and the presence of oxygen causes the biomass to consume the food.
Optimizing treatment process
By adjusting the conditions, biomass can be optimized to meet required treatment standards. Parameters which can be adjusted include
1. the duration for which the biomass is present in the aerated tank (known as sludge age or solids retention time (SRT
2. dissolved oxygen (DO concentration
3. food to microorganism (F/M ratio.
The sludge age is key as it enables the correct biomass flora. By manipulating a high F/M ratio at the inlet of the aerated tank, a selection of well-settling, floc-forming bacteria over poor-settling filamentous bacteria can be encouraged - critical to ensuring effluent quality.
Tackling complex nutrient removal
Typical biological wastewater treatment is designed to achieve effluent quality standards for Biochemical Oxygen Demand (BOD5 , Chemical Oxygen Demand (COD , Total Suspended Solids (TSS and Ammonia (NH3-N . However, there is frequently a need to remove the nutrients Total Nitrogen (TN and Total Phosphorus (TP which, when discharged into the environment, can lead to algal blooms, deoxygenating the water course and causing harm to fish and other water life.
For an activated sludge plant with positive DO and sufficient sludge age, nitrifying bacteria will convert most of the ammonia (NH3-N pollutant load into nitrate (NO3-N by a process of nitrification. However, this does not reduce the TN. By avoiding aeration and providing anoxic conditions, where there is oxygen bound in nitrate rather than DO, denitrifying bacteria will convert nitrate to nitrogen gas (N2 which is then released into the atmosphere, thus reducing TN in the treated effluent.
To enhance TP assimilated in the biomass, anaerobic conditions (where there is no DO or nitrate present are required to boost formation of phosphate accumulating organisms (PAOs with phosphorus release, allowing enhanced uptake of phosphorus in the biomass when in aerobic conditions. Chemicals such as ferric chloride can be added to react with phosphorus to create a ferric phosphate precipitate which is bound in a chemical sludge for removal with the surplus activated sludge.
Configuring activated sludge process
The activated sludge process is a generic treatment process which can be configured depending on the treatment challenge and site requirements. The main activated sludge types are
1. aeration tanks or lanes with final settlement tanks (FSTs
2. oxidation ditches with FSTs
3. sequencing batch reactors (SBRs .
SBRs differ from other activated sludge processes in that they are temporal, whereas others are spatial. With SBRs, all activated sludge treatment stages occur in a single tank in a timed sequence, while other processes require separate aeration tanks, FSTs and RAS lines to undertake these stages. Because the entire SBR area is used for settlement, a larger settlement area than that of FSTs is available, enhancing SBR effluent. SBRs lend themselves to sites where there is limited land available as FSTs are not required and modular wall construction makes the SBR configuration suitable for expansion.
The ICEAS SBR can be configured to achieve treated effluent standards of 10/10/1/5/1 mg/l BOD/TSS/NH3-N/TN/TP respectively, which covers the tightest of Middle East effluent standards while a customized design approach enables customers to overcome complex challenges.
Qoute_1 The most common form of biological treatment is known as activated sludge