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Every wastewater treatment plant spends money getting rid of ammonia. At Severn Trent Water alone, aeration to oxidise ammonia to nitrate costs around £1 million a year, burning through 10 GWh of electricity and generating roughly 5,000 tonnes of CO2 in the process....
A new palm oil mill in Kutai Timur, East Kalimantan, is processing 60 tonnes of fresh fruit bunches an hour and with them, up to 600 cubic metres a day of palm oil mill effluent (POME) carrying a chemical oxygen demand (COD) of roughly 100,000 g/m³. That's an enormous...
The problem SBI Group runs a landfill site in Victoria, Australia, and needed to deal with residual odour from its landfill gas stream. The gas doesn't arrive clean: it's a lean, wet mixture — 25% to 50% methane by volume, saturated with moisture, carrying 0% to 5%...
Ammonia production accounts for between 3% and 5% of the world’s natural gas production. This consumption equates to between 1% and 2% of the world’s energy supply. Here on earth, it requires a great deal of effort to produce ammonia. The following article, paradoxically, concerns efforts to remove it from our wastewaters using thermal ammonia stripping.
Within our solar system, there is abundant ammonia, spread throughout the planets. Astrogeologists estimate there are approximately 220 million square kilometres of sub-surface ammonia-water oceans on 14 moons and Pluto. One on Titan is estimated to have a surface area of 80 million square kilometres. This area compares with the oceans on earth, which cover 361 million square kilometres. A 100 km high plume of ammonia-rich gas was found on Jupiter, near the Great Red Spot, by the NASA spacecraft Juno in 2016. Indeed, it is possible that ammonia gives the Great Red Spot its distinctive colour.
Ammonia is also considered to be an essential ingredient to the origins of life on earth over 4 billion years ago. A team of scientists, working at the Carnegie Institution’s Geophysical Laboratory in Washington, concluded that one of the necessary first steps for life to begin the conversion of nitrogen to ammonia might have occurred in deep ocean hydrothermal vents.
Apart from being a necessary ingredient for life itself, there is a multitude of uses for ammonia and ammonia by-products. For example, the textile industry uses ammonia in the dyeing of wool, cotton, and silk as well as in the production of nylon. Household floor cleaners and detergents commonly use ammonia. Similarly, process industries use ammonia for pH control, as well as the management of NOx. NOx, when combined with ammonia in the correct conditions, produces nitrogen and water.
However, despite the undoubted need for ammonia in our lives, when it permeates through to parts of the environment where it is unwanted, it becomes a source of damage to human health and ecosystems. In many countries, authorities classify ammonia as an extremely hazardous substance. In the atmosphere, gaseous ammonia reacts with other pollutants to form tiny particles of ammonium salts that degrade air quality and, by affecting breathing, harm human health.
One of the primary naturally occurring sources of ammonia here on earth is from the decaying of organic matter. Ammonia forms during the degradation of amino acids within acidogenesis. It also forms part of the excreta cycle of humans and animals, as the kidneys secrete ammonia to neutralize excess acid. Consequently, it is a commonly encountered water pollutant.
Although ammonia is an essential source of nutrient for bacterial growth during anaerobic digestion (AD), its inhibitory effect at high concentrations can be lethally toxic to bacteria that have benefitted from its presence at lower concentrations. With an increasing global interest in producing biogas from food waste, the difficulties encountered with ammonia poisoning of AD facilities are becoming more frequently encountered.
Protein‐rich substrates provide a sound base for methane production. They are of great interest in commercial biogas production. Unfortunately, high loadings with such materials often correlate with process instability due to the presence of ammonia released during the acidogenic phase of AD.
The same issues arise within landfill sites. In Hong Kong, the high loadings of protein in the waste streams entering the landfills, namely meat and meat products, have resulted in high ammonia concentrations within the landfill leachate. Readings of up to 6,000 ppm are not uncommon. In the UK, during the outbreak of bovine spongiform encephalopathy, otherwise known as mad-cow disease, it was necessary to dispose of animal carcasses within sanitary landfills. In such sites, leachate contained ammonia concentrations of up to 9,000 ppm.
Figures vary, but as ammonium ion concentrations increase in an anaerobic digester, typically above 1000 ppm, performance in terms of biogas production, drops off. Full inhibition of AD occurs at around 5000 ppm. It is, therefore, a crucial requirement to manage ammonia concentrations, a requirement for which there exists a wide range of options.
In the past, the most commonly employed methods used with AD plant have been to lower the pH, to decrease the free ammonia concentration, or to dilute the digester contents with water. It is also possible to add lignocellulosic biomass, with a high C:N ratio, to increase the C:N ratio of the substrate in the digester.
Where such approaches are not possible, or not desirable for process efficiency considerations, there are also several technology variants that operators can deploy to control ammonia. Such approaches may also be employed to manage landfill leachate. It is not so much a lack of choice, which is the issue here, but rather an understanding of the issues that each option raises.
Biological nitrification is widely employed, well understood, and generally reliable. Biological nitrification produces varying amounts of sludge and requires both oxygen and carbon to perform effectively. The process requires large holding volumes, as well as significant air (oxygen) and carbon-source additions, subject to the organic carbon and ammoniacal nitrogen loadings involved. The biological process may also produce nitrous oxide (N2O), a potent greenhouse gas. N2O has a global warming potential 265–298 times that of CO2. On average, N2O emitted today remains in the atmosphere for more than 100 years.
In 1995 researchers discovered that Anammox, a previously unknown bacterium, was converting ammonia directly into N2 in a fluidized bed reactor. The process subsequently developed does not require carbon and produces less sludge than the classic nitrification/denitrification process. Anammox bacteria are specialised and slow-growing, which in turn leads to increased operational risk. Start-up can often take several months.
With this technology, ammonia passes through a membrane into an ionic fluid, driven utilizing electrical power. The process includes the possibility to recover ammonia from the fluid or to convert it to nitrogen gas. This technology is an attractive option, providing the potential for a compact and effective ammonia removal system. One study has reported that it requires 4 kWh per pound of ammonia removed by this method. A facility removing 4.5 tonnes per day would, therefore, require 1.87 MW of electricity or 44.8 MWh per day.
Ammonia diffuses through a hydrophobic membrane into sulfuric acid under osmotic pressure. It is necessary to increase the wastewater pH needs c. ph10, leading to notable chemical consumption. In many situations, the process must include pH reduction with the addition of acid. Where an operator can accommodate the logistics and cost of managing chemicals on site, this may be a viable option to consider.
This form of air stripper requires pH adjustment to above pH10. This requirement results in issues like those of membrane contactors. A substantial airflow is necessary to achieve stripping by this means, usually in the range of 3000:1, air to wastewater. A facility treating 100 cubic meters per hour of wastewater requires an airflow rate to the order of 300,000 cubic meters per hour, making for some pretty substantial engineering works. pH-driven air stripping, being a physicochemical process, can provide reliable ammonia removal after reaching operating conditions.
This approach to ammonia removal requires the addition of heat to raise the temperature of the wastewater stream, which leads to high operational costs where waste heat is not available. The process can achieve 98.5% removal. In many situations, the process does not need pH adjustment, subject to incoming pH, and alkalinity. The airflow requirement is substantially less than that required for pH driven air stripping.
Thermal ammonia stripping and pH-driven ammonia stripping employ similar techniques to achieve ammonia removal. Air-stripping is itself a simple desorption process. Heat or the addition of a base breaks the ammonium ion bond. In the case of pH-driven ammonia stripping, the process requires considerably more air, perhaps as much as ten-fold. However, the process is not as temperature-dependent as thermally driven air stripping, where a drop in temperature can bring the processes involved to a halt.
There are two forms of ammonia encountered in wastewater. The ionic form (NH4+) and the gaseous form (NH3). One may write the equation governing the relationship between ammonia gas and the ammonium ion as follows:
NH4+ + OH- ↔ NH3 + H2O
Dissociated ammonia ion (NH4+) is converted to undissociated ammonia gas (NH3) by the addition of a base (OH-), such as sodium hydroxide. As the temperature of the water increases, so the amount of free ammonia gas also increases. The balance of this equation is a function of pH and temperature. Low pH and low temperature push the balance towards NH4+.
The ratio of ammonia in the gas phase to the total ammoniacal nitrogen, referred to as ‘f,’ may be expressed as follows:
f = [NH3] / [NH3] + [NH4+]
The relationship between pH, temperature, and ‘f ‘takes the general form represented in below.
As the temperature increases, represented by the direction of the arrow above , the necessary pH to maintain a value of ‘f’ decreases. This relationship provides the fundamental functionality of the thermal ammonia stripper: to remove the need to increase the pH while maintaining performance.
The thermal ammonia stripping process involves pre-heating of the wastewater, the passage of the wastewater counter-current to airflow in the stripper column, to remove ammonia in the wastewater. In the waste-gas approach, a thermal oxidiser destroys the ammonia in the ammoniated air. The heat generated within the thermal oxidiser in turn powers the process.
The systems developed in Hong Kong are for discharge flows. The West New Territories (WENT) landfill facility receives its thermal energy from landfill gas. With a design flow rate of 1,800 m3/day, recently upgraded to 3,350 m3/day, as much landfill gas as necessary was available for use. The design duty for this first plant was with an influent of 6,700 ppm ammonia for reduction to a discharge of 100 ppm.
The WENT facility now removes 14.5 tonnes of ammonium ion per day. Subsequently, operators installed similar processes on six additional sites around Hong Kong. A good example is a unit operated by at the South East New Territories (SENT) landfill site
Improvements in thermal efficiency, coupled with a deeper understanding of the processes at work, has resulted in the technology becoming less Hong Kong specific, making it suitable for adaptation for use in a range of varied applications in other countries. One primary interest is its use within anaerobic digestion to control ammonia within the process, as well as to prevent its release in discharge flows.
Within a typical AD facility, there are four locations where reduction or removal of ammonia may be possible.
Research into the feasibility of removing ammonia during or after the hydrolysis-fermentation stage has resulted in limited success. The practical options are within the digester itself, in a recycle flow or from the effluent.
Stripping ammonia within the digester vessel leaves limited scope for process control. Several researchers have completed work using biogas as a stripping medium. With low-strength ammonia, this may be an option to consider.
The main opportunities for ammonia control in large-scale commercial facilities are, therefore, in recycle and discharge flows. The former impacts the AD process and leads to improved performance. The latter is a matter of discharge compliance.
In the industrial sector dealing with ammonia removal from wastewater, there are no perfect solutions. Each technology has its point of optimum application. In the case of thermally-driven ammonia stripping, there are several attributable benefits which can help indicate situations of optimum deployment:
Where waste heat is available in the form of steam, or heat from engine exhausts, for example, for the ammonia removal process, after ammonia stripping from wastewater there is a requirement to remove ammonia gas from the stripping air.
More recently, Organics has developed a process that facilitates the recovery of either ammonium hydroxide or anhydrous ammonia. This approach further develops two key process themes: employing waste heat and avoiding the use of chemicals. The Process Flow Diagram (PFD) for a system recovering ammonia is provided below. Clean, cold water is used to remove ammonia from stripping air as ammonium hydroxide. Additional concentration and separation make possible the formation of anhydrous ammonia.
One possibility that may prove interesting, as we move into the non-fossil fuel age, is the use of ammonia as a fuel. The energy content of liquid ammonia is 11.5 MJ/L, or approximately 30% that of diesel. Fuel-cells can use ammonia to produce power, which offers the potential for a local, revenue-generating means of disposal. Operators can use ammonia as a fuel in engines and turbines. During World War II ammonia was used to power buses in Belgium. A high-octane rating of 120 and low flame temperature permits the use of high compression ratios without the penalty of high NOx production. Another advantage is that, since ammonia contains no carbon, its combustion cannot produce carbon dioxide, carbon monoxide, hydrocarbons, or soot. However, because it’s flame speed is slow (only one-fifth that of methane), it is challenging to use as a fuel.
Ammonia is joining a growing list of substances that need active prevention from polluting the environment and, where practical, should be recycled. Using waste heat to meet these objectives assists with ensuring a long-term sustainable solution to the challenge of ammonia disposal.
Organics has designed thermal ammonia stripping systems since 1997
Every wastewater treatment plant spends money getting rid of ammonia. At Severn Trent Water alone, aeration to oxidise ammonia to nitrate costs around £1 million a year, burning through 10 GWh of electricity and generating roughly 5,000 tonnes of CO2 in the process....
A new palm oil mill in Kutai Timur, East Kalimantan, is processing 60 tonnes of fresh fruit bunches an hour and with them, up to 600 cubic metres a day of palm oil mill effluent (POME) carrying a chemical oxygen demand (COD) of roughly 100,000 g/m³. That's an enormous...
The problem SBI Group runs a landfill site in Victoria, Australia, and needed to deal with residual odour from its landfill gas stream. The gas doesn't arrive clean: it's a lean, wet mixture — 25% to 50% methane by volume, saturated with moisture, carrying 0% to 5%...
Hong Kong's NENT landfill already runs two ammonia stripping lines to treat leachate — the liquid that percolates through buried waste. Growing volumes meant those two lines needed a third, independent treatment train, sized to handle up to 1,500 m³/day of leachate...
Veolia's NENTX landfill extension near Fanling, Hong Kong needed an ammonia stripping plant (ASP) capable of handling 2,000 m³/day of leachate, cutting ammonium (NH4) from 4,500 mg/l down to under 200 mg/l, on a site where land is expensive and the climate swings from...
