Key Highlights
- Sidestream (also spelled side stream in some published literature) treatment refers to removing or diverting a portion of the wastewater from the wastewater treatment processes to remove ammonium and phosphate from the wastewater.
- The SHARON and the ANNAMOX processes are sidestream treatment technologies discussed by the EPA.
- Other sidestream treatment technologies are the BABE process, coagulation, flocculation, and the use of granular activated sludge.
- Sidestream treatment consumes less energy to remove ammonium and phosphate compared to wastewater treatment not using sidestream treatment.
- Refitting the infrastructure of wastewater treatment plants to accommodate sidestream treatment can be challenging and expensive.
In a wastewater treatment plant, the water resulting from membrane treatment and in the supernatant liquid from sludge digestion is often high in ammonium (NH4+) and phosphate species. Subsequent treatment processes do not always remove these high concentrations and on the other hand, there are regulatory limits for total nitrogen (includes ammonium) and total phosphorus (includes phosphate).
What can be done to remove ammonium and phosphate from the water so that the discharged water complies with the regulations? The answer is sidestream treatment. But what exactly is sidestream treatment and how is it done?
What is sidestream treatment?
Sidestream (also spelled side stream in some published literature) treatment refers to removing or diverting a portion of the wastewater from the wastewater treatment processes. This removed water is treated before it is returned to the headworks of the wastewater treatment plant where it is combined with the normal influent. Sidestreams can originate from membrane treatment and as supernatant liquid from sludge digesters.
This removed or diverted portion of the wastewater, also known as reject water, is the dirtiest, most concentrated in water contaminants and has the highest concentration of biological oxygen demand (BOD). Specifically this water is very high in nutrients including ammonium and phosphate.
What is the purpose of sidestream treatment?
The main purpose of sidestream treatment is to remove ammonium and phosphate from the wastewater. Ammonium and phosphate, if left untreated in the wastewater treatment process, can enter the environment from the discharged water and compromise the water quality, causing eutrophication and health issues.
The other concern is regulations – there are regulatory discharge limits for total nitrogen (contains ammonium) and total phosphorus (contains phosphate) set by the EPA in the effluent limitations guidelines and in the National Pollutant Discharge Elimination System (NPDES). To comply with the regulatory limits in the discharged water for total nitrogen and total phosphorus, it is essential for wastewater treatment plants to remove ammonium and phosphate from the wastewater and sidestream treatment is a practical way to achieve this.
Where is sidestream treated water used?
The most common application of sidestream treated water is its use in cooling towers and in irrigation. Microorganisms thrive on ammonium and phosphate for their growth and with these two chemical species removed from the sidestream treated water, the buildup of microorganisms does not occur in applications, thus preventing abrasion and corrosion of components and toxicity concerns. Specifically, sidestream treated water does not cause abrasion and corrosion in cooling towers nor eutrophication and crop damage when used in irrigation.
How does sidestream treatment improve nutrient removal?
Sidestream treatment can treat the water to remove ammonium and phosphate before the treated water is returned to the headwork of the publicly owned treatment works such as in municipal wastewater treatment plants.
What technologies are used for sidestream treatment?
Some of the ammonium-removing technologies from sidestreams discussed by the EPA are the SHARON and the ANNAMOX processes. These processes involve microorganisms.
The SHARON process
The SHARON (Single reactor for High activity Ammonia Removal Over Nitrite) process occurs in a completely mixed bioreactor to treat sidestreams from sludge digestion that contain high concentration of ammonium (NH4+). In this process, ammonium is converted to nitrite (NO2-) and then to nitrogen gas.
The conversion of ammonium to nitrite, illustrated below, is an exothermic process, occurring at a temperature between 30oC to 40oC and involves oxygen (O2) and oxidizing bacteria:
Two types of bacteria are involved – ammonia oxidizers that convert ammonium to nitrite and nitrite oxidizers that convert nitrite to nitrogen gas. The SHARON process is used to stop the conversion of ammonium at the nitrite step using elevated temperature that affect these bacteria.
At higher temperatures, the ammonia oxidizers grow faster than the nitrite oxidizers. What this means is that at higher temperatures, because there are more ammonia oxidizers (because they grow faster) and relatively less nitrite oxidizers, more of the ammonium is converted to nitrite and less of the nitrite is converted to nitrogen gas. Therefore, the use of higher temperature in the SHARON process is used to control the amount of nitrite and nitrogen gas produced.
The hydraulic retention time depends on the ammonium concentrations in the sidestream that needs to be treated for regulatory compliance. This retention time may be one to two days.
The ANNAMOX process
The ANNAMOX (ANaerobic AMMonium OXidation) process converts the ammonium-nitrite mixture from the SHARON process to nitrogen gas (N2). What does this mean and how does it happen?
The ANNAMOX process occurs after a specialized form of the SHARON process. Recall that the SHARON process controls the amount of nitrite produced from ammonium using high temperatures. Here, the specialized form of the SHARON process is this – to convert only 50% of the ammonium to nitrite. This is done by limiting the oxygen supply in the SHARON process.
The ammonium-nitrite mixture (mixture of NH4+and NO2-) produced in the SHARON process is then used in the ANNAMOX process where it is converted to nitrogen gas (N2) and water (H2O) is produced. This reaction is illustrated as follows:
The bacteria involved in the ANNAMOX process have a slow growth rate – they double in number in 10 days at 30oC. A sufficient volume of these bacteria must be available to ensure the ANNAMOX process.
Other technologies for sidestream treatment
Other technologies for sidestream treatment centers mostly on enhancing the concentration of microorganisms for the SHARON and the ANNAMOX processes for the removal of ammonium via its conversion to nitrite. These technologies are discussed as follows.
The BABE process
Besides the SHARON and the ANNAMOX processes, the EPA also mentioned a third process for ammonium removal – the BABE (BioAugmentation Batch Enhanced) process. The sidestream containing ammonium is mixed with the return activated sludge from the main biological treatment process in the wastewater plant with one purpose – to augment the bacteria in the settled sludge – specifically to grow both nitrifying and denitrifying bacteria so that they can treat the ammonium in the sidestream.
Coagulation and flocculation
Coagulation and flocculation have been used to recover the ammonium species from the sidestream. In a published study, researchers optimized the physical-chemical process of coagulation and flocculation to recover ammonium while also eliminating organic and suspended matter from the sidestream. The use of 25 mg/L of ferric chloride and a flocculant composed of silicon, aluminum, and iron salts along with one minute of rapid mixing at 200 rpm, slow mixing for 30 minutes at 30 rpm, followed by another 30 minutes for the settling led to a 95.2% removal of total suspended solids and a high ammonium recovery rate of 570.6 mg NH4+/L.
Aerobic granular sludge
Aerobic granular sludge (AGS), as the name implies, is sludge in the shape of granules. These granules contain microorganism that are involved in degrading chemicals they are in contact with via aerobic (requires oxygen) chemical reactions. AGS has a higher settling velocity compared to flocs due to their high biopolymer content and the presence of microorganisms that provide high density.
In a published study, researchers used AGS to recover phosphorus from sidestream water. In the set-up, AGS was added in a reactor where nitrification, denitrification, and phosphorus removal processes occurred simultaneously. Although the use of AGS led to no distinct improvement in ammonium removal, phosphorus removal was high – up to 95% total phosphorus was removed.
What are the benefits and challenges of sidestream treatment?
One of the benefits of sidestream treatment is that it is more resource friendly and it consumes less energy than other wastewater treatment processes. This point can be highlighted by the use of the combined SHARON -ANNAMOX processes – as explained above – that can result in 90% to 95% overall nitrogen removal from the sidestream. The process operates under low oxygen concentrations and does not require an external carbon source.
Sidestream treatment also helps deal with production variables because contaminant-concentrated water is removed before it reaches the wastewater treatment plant. In the SHARON process, temperature and oxygen levels are used as controlling variables to control the amount of nitrite formed. This is important to treat the ammonium and the nitrite in the sidestream before the clean water goes back into the headwork of the wastewater treatment plant.
However like other technologies that have their benefits and challenges, sidestream treatment also has its own challenges. Refitting the infrastructure of wastewater treatment plants to accommodate sidestreaming treatment can be challenging and expensive.
Sidestream treatment is a simple solution yet with sophisticated methods involving microorganisms to remove ammonium and phosphate from the water so that the discharged water is in compliance with federal and state regulations. However the rise in concerns about microbial resistance due to the increasing overload of pharmaceuticals in wastewater is a concern – how will this issue affect the performance of the microorganisms involved in sidestream treatment?
Take a quiz about sidestream treatment
About the Author
Saleha Kuzniewski
Saleha Kuzniewski, Ph.D. has authored several publications in the fields of scientific research, biotechnology, and environmental regulations. She is the winner of the 2023 Apex award for publication excellence. She is also the founder of Environmental Remediation & Innovations, LLC. Kuzniewski can be reached at [email protected].