Inhibition

5-C/N ratio of the substrate

For an efficient biomass growth and stable and sustainable process in biogas plants, carbon/nitrogen (C/N) ratio of the substrate should be around 20-30/1. Too high C/N ratio leads to inadequate nitrogen for biomass formation and low treatment efficiency. Too low C/N ratio causes ammonia and ammonium accumulation in the system and inhibition of the methanogen activity. To balance the C/N ratio of the substrate around the desired range in anaerobic systems, carbon rich or nitrogen rich wastes can be mixed with the wastes that`s lack off the other one. For example wheat straw, food wastes or carbon rich wastes can be mixed with the animal manure such as pig and chicken manure that has C/N ratio around 8-15.

6-Content of substrate and organic loading rate

Substrate components are important to maintain adjusting the HRT, temperature of the system and the predigestion methods in anaerobic systems. Lignocellulosic biomass, such as agricultural residuals and energy crops accumulate from agricultural, forestry, municipal, and other activities are an abundant organic sources for biogas and methane production. Main content of the lignocellulosic biomass consist of three types of polymers:cellulose, hemicellulose, and lignin. Although they are rich in carbonaceous source, their structural and chemical properties make it resistant to biodegradation by enzymes and microbes. These kind of sources should be predigested prior to feeding to the system, or else biogas production rate can slow down.

Organic loading rate (OLR) is the organic material corresponding COD or VSS that is fed to the system per reactor volume and time. OLR has to be well arranged to maintain stability, prevent fluctiations (biogas production rate, treatment efficieny, etc.) and prevent inhibition of VFAs arise from accumulation and rapid pH decreases. Sudden over loadings cause the anaerobic process failures and time wastes.
  

7-Inbiting Parameters

• Ammonia: Ammonia is produced by the anaerobic degredation of the protein and urea by the anaerobic microorganisms. At the end of this process nitrogenous compounds are degraded to ionized ammonia  (ammonium- NH4+) and unionized (free) ammonia (NH3). Free ammonia is the actual inhibitor compared to ammonium. Since it can permeate through the cell membrane it can change the intracellular pH and disturb the cell activity causing toxicity. Though there are many different ranges given in the literature on ammonia inhibition concentration, roughly a total amount of ammonia concentration higher than 4 or 5 g/L can be inhibitory to the anaerobic system. Temperature, pH, acclimation can change the level of inhibition. High temperature and pH values cause the ammonium ion turn into free ammonia that is more toxic to the microorganisms. The figure below presents how ammonia and ammonium changed with pH and temperature changes.

• Hydrogen sulfide:  Sulfate is a common endproduct of many industrial wastewater. In anaerobic reactors, sulfate is reduced to sulfide by the sulfate reducing bacteria (SRB). Sulfate reduction is performed by two major groups of SRB including incomplete oxidizers, which reduce compounds such as lactate to acetate and CO2, and complete oxidizers, which completely convert acetate to CO2 and HCO3. As a result sulfate reduction ends with two kinds of inhibition which one of them is due to competition between SRB and methanogens for common organic and inorganic substrates, causing the suppresion of the methane production. And the other one is the inhibition results from the toxicity of sulfide to various bacteria groups.

• Heavy metals and light metals: Heavy metals are mostly found in municipal sewage and sludge in high concentrations, and they can not be degraded by biological consortiums. They are accumulated in the environment and in the microorganism cell leading to toxic effects such as distruption of enzyme functioning and microbiological protein structure. Chromium, iron, cobalt, copper, zinc, cadmium, and nickel are the most abundant heavy metal in municipal wastes. Although heavy metals are part of enzymes that drive many reactions for the cell activity, whether heavy metals cause inhibition or stimulation in the cell depends on the metal concentration in it. Metals in wastewater can be in interaction with the other chemicals: they may precipitate as oxides or sulfides, adsorbate to the other solids and may produce complexes such as intermediates and end products. Of all these form of heavy metals only the free formed, soluble metals are toxic to the microorganisms.
• Salts toxicity has been studied in biological field for a long time. High salt levels cause bacterial cells to dehydrate due to osmotic pressure. Although the cations of salts in solution must always be associated with the anions, the toxicity of salts are determined by the cations. The light metal ions including sodium, potassium, calcium, and magnesium that are naturally present in the wastewater are required for microbial growth. Breakdown of organic materials and chemical addition for pH adjustment contribute to the salts concentration in wastewater treatment plants. While moderate  concentrations stimulate microbial growth, excessive amounts slow down the growth, and even higher concentrations can cause severe inhibition or toxicity.

• Organics: A variety of organic compounds that dissolve almost non in water  have the potential of inbiting the anaerobic process. Those organic substances may be adsorbed on the particulate matter in the digester environment and as a result accumulate on the membrane of the cells causing swelling and leaking, ruin the ion transportation and pH balance of the cell. Finally they cause lysising in the cell. The most common organics that lead to toxicity on anaerobic microorganisms are halogens, phenols, benzenes, aldehydes, double bonds, amines and amides, and aromatic compounds. Only after acclimation periods long enough, biodegradation of these compounds can be possible.

• LCFAs and SCFAs: Long chain fatty acids (LCFA) that are produced by the degradation of the lipids may also cause toxicity in anaerobic digestors if accumulated. Any physical, chemical or biological parameter that may lead to an unbalanced condition in digester can end up with LCFA accumulation. Accumulation of LCFAs cause cell wall destruction in methanogens. Also since LCFAs are lipids and lighter than microorganism flocs, they provoke floatation and biomass washout. Some typical examples of LCFAs are palmitic, stearic, capric, linoleic, and oleic acids.
• Short chain fatty acid (SCFA) such as  acetic, propionic, butyric, isobutyric, valeric, isovaleric, and caproic acids are the end products of biodegradation of complex organic materials. These acids concentrations are measured to determine the health state of an anaerobic system. If SCFA is accumulated because of some reason, the first thing that happens is pH decreases. Consequently methanogenic activity slows down and biogas production rate decreases. If the problem is not a solved, then the anaerobic process may fail. 

References 
1-Dublein, D., Steinhauser, A., Biogas:From Waste And Renewable Energy Resources, Wiley-Vch Verlag GmbH Co. & KGaA, Weinheim, ISBN 978-3-527-31841-4, 2008.
2-Samir K. Khanal, 2008. Anaerobic Biotechnology For Bioenergy Production, Principles and Applications. 2008 John Wiley & Sons, Inc. ISBN: 978-0-813-82346-1.
3-Ye Chen, Jay J. Cheng *, Kurt S. Creamer, Inhibition of anaerobic digestion process: A review, Bioresource Technology 99 (2008) 4044–4064.