Understanding nutrient removal in wastewater treatment

Removal of phosphorus from wastewater

Method

Description

Enhanced biological phosphorus removal (EBPR)

• EBPR is one of the most sustainable and promising methods for wastewater treatment. It is applied to activated sludge systems.

• Phosphate-accumulating organisms or PAOs (found in activated sludge) can absorb and store more phosphorus than other bacteria and uptake phosphorus from wastewater.

• Factors to consider for enhanced biological removal:

1. Carbon availability. The nature of the carbon source determines the efficiency of EBPR and rules the competition between PAO and denitrifiers in the system. 

2. A calibrated process model. A well-calibrated EBPR process model is a vital tool for assessing the viability of EBPR, optimizing design and troubleshooting. 

3. Anaerobic/Aerobic zone volumes. Maintaining adequate anaerobic and aerobic zone volumes is critical for the growth of sensitive PAOs. This is an area of active study as many old assumptions are being challenged. 

4. Swing zones. An EBPR process typically has anaerobic zones, anoxic zones (optional),and aerobic zones. To handle variable conditions, it is important to design flexibility into the system.  Swing zones are zones that can be aerated or unaerated to provide operational flexibility.   

5. Aeration control. Aeration control is critical to enhance the efficiency of EBPR and reduce operating costs. Often blower and aeration systems need to be assessed and modified to achieve target oxygen levels.

6. Impact of phosphorus from sludge recycle streams. The phosphorus compounds recovered from  PAOs can be released in the solids handling processes and return to the start of the treatment process via recycle streams. It is important to consider these nutrient loads and assess side stream treatment processes.   

Chemical precipitation

• It removes phosphorus from wastewater by adding coagulants such as calcium, iron and aluminum salts and removing the resulting metal-bound phosphorus via settling or filtration. 

• Chemical precipitation for phosphorus removal typically has a lower capital cost than  EBPR but often has a higher annual cost when used as a stand-alone system.  

• Chemical precipitation for phosphorus removal is not dependent on carbon availability in the wastewater like EBPR. 

• Factors to consider for chemical precipitation of phosphorus: 

1. Multiple dosing locations. The required chemical dose depends on the phosphorus concentration in wastewater. Studies have shown dosing coagulants at multiple places in the treatment process can reduce the required amount of chemicals compared with a single dosing point. 

2. Chemical options. The most common metal salts used in chemical precipitation are alum (aluminum sulfate), ferric chloride, sodium aluminate, ferrous sulfate, ferric sulfate and ferrous chloride. 

3. Impact on sludge volume. With chemical addition, sludge volume can increase significantly and the impact on the solids handling process should be evaluated. 

Combined biological and chemical phosphorus removal

• When chemical addition is combined with EBPR, it is implemented as a polishing/ backup step, usually in the secondary and tertiary treatment processes.  

• This approach is often used with a biological phosphorus removal process to add resiliency and redundancy.  

Removal of nitrogen from wastewater

Method

Description

Nitrification and denitrification with activated sludge

• The most common method for nitrogen removal in WWTPs is biological (activated sludge) treatment. It involves two steps:

1. Nitrification. Nitrifiers, including nitrite-oxidizing bacteria and ammonia-oxidizing bacteria, convert the total ammonia in wastewater to nitrate. 

2. Denitrification. In an anoxic environment, denitrifiers reduce nitrate and nitrite to nitrogen gas. 

• Though widely used, this approach typically requires large treatment volumes, resulting in large capital costs and footprint. 

Denitrification filters

• These are used as a part of a tertiary process to convert nitrate to nitrogen gas after the secondary process.  

• Denitrification filters are of two types:

1. Downflow denitrification filters. Operates in a conventional filtration mode, consisting of media and support gravel. During the process, nitrate is metabolized to nitrogen gas, which is embedded in the filter media.  

2. Upflow continuous backwash filters. The influent wastewater flows upward through the filter countercurrent to the movement of the sand bed. 

Anaerobic ammonium oxidation

• The autotrophic anaerobes, called anammox, convert ammonia and nitrite into nitrogen gas under anoxic conditions. 

• The anammox process is an area of research for nitrogen removal from ammonium-rich wastewater and side stream treatment. 

Membrane aerated biofilm reactors (MABRs)

• Gas-permeable hydrophobic membranes deliver oxygen directly into the wastewater, with a natural layer of microorganisms forming on the oxygen-rich surface of the membrane.

• Oxygen-rich to depleted profile forms in the microorganism layer, allowing for nitrification and denitrification in one tank.