Environmental Engineering Reference
In-Depth Information
ammonium inhibition when diluted to the extent necessary to stop ammonia inhibition,
and already a washout because of dilution occurs.
• It is also possible to remove ammonium from the digester liquid. This method is usually
most cost effective and rarely used. One such process is stripping ammonia from the liq-
uid. It is also commercially available (GNS 2009).
32.3.2.2.4 pH
In anaerobic digestion the pH affects the most methanogenic stage of the process. The optimum
pH 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, serious actions to stop process failure must be taken. When
using UASB systems (or other systems with granule-like microorganisms), which utilize liquid
substrates with a low suspended solids concentration, the normal pH of operation is 6.9-7.1. In such
cases, the pH limit of successful operation is 6.7.
In normally operated digesters, there are two buffering systems that ensure that pH stays in a
desirable range:
• Carbon dioxide-hydrogen carbonate-carbonate buffering system. During digestion CO
2
is continuously produced and released into gaseous phase. When pH value decreases, CO
2
is dissolved in the reactor solution as uncharged molecules. With increasing pH value, dis-
solved CO
2
forms carbonic acid which ionizes and releases hydrogen ions. At pH 4 all CO
2
is in form of molecules and at pH 13 all CO
2
is dissolved as carbonate. The center point
around which pH value swings with this system is at pH 6.5. With concentrations between
2500 and 5000 mg/L, hydrogen carbonate gives strong buffering.
• 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 center point around which pH value swings with this system
is at pH 10.
Both buffering systems can be overloaded by the feed of rapidly acidifying (quickly degradable)
organic matter, toxic substances, decrease of temperature, or a too high loading rate to the reactor.
In such case, a pH decrease is observable, combined with CO
2
increase in the biogas. Measures to
correct the excessive acidification and prevent a process failure are as follows:
• Stopping the reactor substrate supply for the time to methanogenic bacteria 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 7.4
and in granule microorganism systems this pH value is 7.0.
• 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 (increasing the residence time
of the substrate).
• Increasing the buffering potential of the substrate. By the mixture of certain substrates that
contain some alkaline substances, the buffering capacity of the system can be increased.
• Addition of the neutralizing substances. Typical are lime (CaO or Ca(OH)
2
), sodium car-
bonate (Na
2
CO
3
), or sodium hydrogen carbonate (NaHCO
3
), and in some cases sodium
hydroxide (NaOH). However, with sodium substances, precaution must be practiced
because sodium inhibition can occur with excessive use.
