Stripping Ammonia in Anaerobic Digesters

Robert Eden discusses thermal stripping in Hong Kong

GIVEN its high population density, coupled with the demanding environmental standards enforced by the Environmental Protection Department, it is not surprising that Hong Kong served as the incubator for our process to remove ammonia from wastewater. What has resulted is a novel thermal ammonia stripping technology, which has low operating costs, high standards, and a small footprint. 

Since 1997, the technology has been applied to both leachate from landfill sites, as well as the side-stream flows from an anaerobic digester (AD). While highly focussed on the needs of Hong Kong, the technology is gaining momentum in other countries, where environmental compliance is a serious issue to address. It is also moving from leachate cleanup applications into AD. 

To many engineers not involved with AD, the problems caused by ammonia may come as a surprise. It is well known that ammonia is toxic to fish and humans (sniff that floor cleaner if in doubt), so it should not be unexpected that many bacteria are similarly distressed.

The problem

Although ammonia is a source of nutrient for bacterial growth during AD, its inhibitory effect at high concentrations can be lethally toxic to bacteria that have thrived on its presence at lower concentrations.

According to the National Non-Food Crops Centre (NNFCC), there are now 486 operational AD plants in the UK, with a further 343 under development. AD is established as a commercially viable form of renewable energy generation. The AD process produces biogas, consisting of methane and carbon dioxide, as well as various trace gases. Biogas can be used directly as fuel, in spark-ignition gas engines or upgraded to natural gas-quality biomethane. The nutrient-rich digestate produced can be used as fertiliser.

With an increasing global interest in producing biogas from food waste, the difficulties encountered with ammonia poisoning of AD facilities are now becoming more frequently encountered.

Protein-rich substrates are valuable 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 from the degradation of amino acids during acidogenesis.

There are two forms of ammonia encountered in wastewater. The ionic form (NH₄⁺) and the gaseous form (NH₃). They are related by the chemical formula:

NH₃ + H⁺ ↔ NH₄⁺

Both forms can, directly and indirectly, cause inhibition in an AD system, although NH₃ is generally recognised to be the main inhibitor. The balance of this equation is a function of pH and temperature. Low pH and low temperature push the balance towards NH₄⁺.

Figures vary, but as ammonium ion concentrations increase in an anaerobic digester, typically above 1,000 mg/L, performance, in terms of biogas production, drops off. Anaerobic digestion is fully inhibited at around 5,000 mg/L. It is, therefore, an essential requirement to manage ammonia concentrations, a requirement for which there exists a wide range of options. 

In the past, the most commonly-employed methods 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 may be deployed to control ammonia. It is not so much a lack of choice, which is the issue here, but rather an understanding of the issues that may be encountered with each option.