**2.2.3 C:N ratio and ammonium inhibition**

In microorganism biomass the mass ratio of C:N:P:S is approx. 100:10:1:1. The ideal substrate C:N ratio is then 20-30:1 and C:P ratio 150-200:1. The C:N ratio higher than 30 causes slower microorganisms multiplication due to low protein formation and thus low energy and structural material metabolism of microorganisms. Consequently lower substrate degradation efficiency is observed. On the other hand, the C:N ratio as low as 3:1 can result in successful digestion. However, when such low C:N ratios and nitrogen rich

Anaerobic Treatment and Biogas Production from Organic Waste 9

In anaerobic digestion the pH is most affecting the methanogenic stage of the process. pH optimum for the methanogenic microorganisms is between 6.5 and 7.5. If the pH decreases below 6.5, more acids are produced and that leads to imminent process failure. In real digester systems with suspended biomass and substrate containing suspended solids, normal pH of operation is between 7.3 and 7.5. When pH decreases to 6.9 already serious actions to stop process failure must be taken. When using UASB flow through systems (or other systems with granule like microorganisms), which utilize liquid substrates with low suspended solids concentration normal pH of operation is 6.9 to 7.1. In such cases pH limit

In normally operated digesters there are two buffering systems which ensure that pH

 Carbon dioxide - hydrogen carbonate - carbonate buffering system. During digestion CO2 is continuously produced and release into gaseous phase. When pH value decreases, CO2 is dissolved in the reactor solution as uncharged molecules. With increasing pH value dissolved CO2 form carbonic acid which ionizes and releases hydrogen ions. At pH=4 all CO2 is in form of molecules, at pH=13 all CO2 is dissolved as carbonate. The centre point around which pH value swings with this system is at pH=6.5. With concentrations between 2500 and 5000 mgL-1 hydrogen carbonate gives

 Ammonia - ammonium buffering system. With decreasing pH value, ammonium ions are formed with releasing of hydroxyl ions. With increasing pH value more free ammonia molecules are formed. The centre point around which pH value swings with

Both buffering systems can be overloaded by the feed of rapidly acidifying (quickly degradable) organic matter, by toxic substances, by decrease of temperature or by a too high loading rate to the reactor. In such case a pH decrease is observable, combined with CO2 increase in the biogas. Measures to correct the excessive acidification and prevent the

 Stop the reactor substrate supply for the time to methanogenic archaea can process the acids. When the pH decreases to the limit of successful operation no substrate supply should be added until pH is in the normal range of operation or preferably in the upper portion of normal range of operation. In suspended biomass reactors this pH value is

 If procedure from the point above has to be repeated many times, the system is obviously overloaded and the substrate supply has to be diminished by increasing the

 Increase the buffering potential of the substrate. Addition of certain substrates which some contain alkaline substances to the substrate the buffering capacity of the system

 Addition of the neutralizing substances. Typical are slaked lime (Ca(OH)2), sodium carbonate (Na2CO3) or sodium hydrogen carbonate (NaHCO3), and in some cases sodium hydroxide (NaOH). However, with sodium substances most precaution must

be practiced, because sodium inhibition can occur with excessive use.

7.4 in granule microorganisms systems this pH value is 7.0.

**2.2.4 pH** 

of successful operation is 6.7.

persists in the desirable range:

strong buffering.

this system is at pH=10.

process failure are following:

can be increased.

residence time of the substrate.

substrates are applied (that is often the case using animal farm waste) a possible ammonium inhibition must be considered. Ammonium although it represents an ideal form of nitrogen for microorganisms cells growth, is toxic to mesophilic methanogenic microorganisms at concentrations over 3000 mgL-1 and pH over 7.4. With increasing pH the toxicity of ammonium increases (Fig. 2).

Fig. 2. Ammonium nitrogen toxicity concentration to methanogenic microorganisms

Thermophilic methanogenic microorganisms are generally more sensitive to ammonium concentration. Inhibition can occur already at 2200 mgL-1 of ammonium nitrogen. However the ammonium inhibition can very much depend on the substrate type. A study of ammonium inhibition in thermophilic digestion shows an inhibiting concentration to be over 4900 mgL-1 when using non-fat waste milk as substrate (Sung and Liu 2003).

Ammonium inhibition can likely occur when digester leachate (or water from dewatering the digested substrate) is re-circulated to dilute the solid substrate for anaerobic digestion. Such re-circulation must be handled with care and examined for potential traps such as ammonium or other inhibitory ions build up.

To resolve ammonia inhibition when using farm waste in anaerobic digestion several methods can be used:

