**2.2.4 pH**

8 Management of Organic Waste

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

Fig. 2. Ammonium nitrogen toxicity concentration to methanogenic microorganisms

over 4900 mgL-1 when using non-fat waste milk as substrate (Sung and Liu 2003).

ammonium or other inhibitory ions build up.

methods can be used:

farm waste in such case.

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

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

To resolve ammonia inhibition when using farm waste in anaerobic digestion several

 First possibility is carefully combining different substrates to create a mixture with lower nitrogen content. Usually some plant biomass (such as silage) is added to liquid

 Second possibility is diluting the substrate to such extent, that concentration in the anaerobic digester does not exceed the toxicity concentration. This method must be handled with care. Only in some cases dilution may be a solution. If the substrate requires too much dilution, a microorganisms washout may occur, which results in process failure. Usually there is only a narrow margin of operation, original substrate causes ammonium inhibition, when diluted to the extent necessary to stop ammonia

 It is also possible to remove ammonium from the digester liquid. This method is usually most cost effective but rarely used. One of such processes is stripping ammonia from

inhibition, and already a washout due to dilution occurs.

the liquid. It is also commercially available (GNS 2009).

ammonium increases (Fig. 2).

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 of successful operation is 6.7.

In normally operated digesters there are two buffering systems which ensure that pH persists in the desirable range:


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 process failure are following:


Anaerobic Treatment and Biogas Production from Organic Waste 11

Block scheme of anaerobic digestion (Fig. 3) shows that technological process of typical anaerobic digestion. It consists of three basic phases: i) substrate preparation and pretreatment, ii) anaerobic digestion and iii) post treatment of digested material, including

**Toxicity to adopted microorganisms3 [mgL-1]** 

**[mgL-1]** 

Cr3+ 130 260 Cr6+ 110 420 Cu 40 170 Ni 10 30 Cd 70 600 Pb 340 340 Zn 400 600 1As inhibitory concentration it is considered the first value that shows diminished biogas production and as toxic concentration it is considered the concentration where biogas production is diminished by

**Metal Inhibition start1**

Table 5. Inhibitory and toxic concentrations of heavy metals

biogas use. In this section all of the processes will be elaborated in detail.

Fig. 3. Block scheme of anaerobic digestion and biogas/digestate utilisation

In general, all types of biomass can be used as substrates as long as they contain carbohydrates, proteins, fats, cellulose and hemicellulose as main components. It is however

**3. Anaerobic digestion technologies** 

70 %.

**3.1 Pretreatment** 
