Dear Les,

I may suggest the following methods:

1-ORP (Oxidation-Reduction-Potential) was used as the controlling parameter to precisely regulate air injection to the sulfide oxidizing unit (SOU). The microaeration technique provided just enough oxygen to partially oxidize sulfides to elemental sulfur without inhibiting methanogenesis. The SOU was equipped with a diffuser at the bottom for the dispersion of sulfide-laden biogas and injected air throughout the column. The SOU can be operated as a standalone unit or coupled with an anaerobic digester to simultaneously remove sulfide from the biogas and effluent. The integrated system was capable of reducing hydrogen sulfide in biogas from 2,450 to less than 2 ppmV with minimal sulfate production at the highest available sulfide loading rate of 0.24 kg/m3 -day. More than 98% of sulfide removed was recovered as elemental sulfur. However, the standalone SOU was able to operate at high hydrogen sulfide loading of 1.46 kg/m3 -day at inlet sulfide concentration of 3000 ppmV and reduce the off-gas hydrogen sulfide concentrations to less than 10 ppmV. The experiment also revealed that the

ORP controlled aeration was sensitive enough to prevent oxygen overdosing (dampening effect) during unexpected surges of aeration. 

For more information on this method, please use the following link:

http://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1765&context=etd

2-Hydrogen sulphide removal in the anaerobic digestion of sludge by micro-aerobic processes: pilot plant experience.

Water Sci Technol. 2009;60(12):3045-50. doi: 10.2166/wst.2009.738.

Hydrogen sulphide removal in the anaerobic digestion of sludge by micro-aerobic processes: pilot plant experience.

Fdz-Polanco M1, Díaz I, Pérez SI, Lopes AC, Fdz-Polanco F.

H(2)S removal from biogas produced in anaerobic digestion of sludge through the introduction of oxygen under micro-aerobic conditions is studied. Research was carried out in two pilot plant reactors (working volume, 200 L each) treating sludge from WWTP with HRT of 20 days. Mixing was provided via sludge or biogas recirculation. Introduction of very low oxygen flow (0.013-0.024 L/L(reactor) d) successfully removed H(2)S content in biogas with an efficiency above 99%. Reactor performance during micro-aerobic operation in terms of biogas production, methane yield and COD removal were not affected by the amount of oxygen supplied, remaining stable and similar to the anaerobic behaviour. Sludge recirculation ( approximately 50 L/h) and biogas recirculation ( approximately 3.5 L/min) as mixing methods were found not significant in H(2)S removal from biogas while biogas recirculation reduced by 10 times dissolved sulphide concentration compared to sludge recirculation.

For more information on this technique, please use the following link:

http://www.ncbi.nlm.nih.gov/pubmed/19955627

3- Robustness of the microaerobic removal of hydrogen sulfide from biogas

I. Díaz, M. Fdz-Polanco

Published March 2012, 65 (8) 1368-1374; DOI: 10.2166/wst.2012.013

Several disturbances presented in full-scale digesters can potentially affect the efficiency of the microaerobic removal process. This study evaluates the variation of the sulfur load and the performance of the system in situations of oxygen lack or excess and after normal rates are recovered. The process was shown to recover from oxygen lack or excess within 28 h when the original conditions were restored in a pilot-plant digester of 200 L treating sewage sludge with HRT of 20 days. The decrease of the sulfur load to the digester did not affect the biogas composition in the short-term and when oxygen rate was reduced to adjust to the lower hydrogen sulfide production, the removal proceeded normally with a lower unemployed oxygen amount. The digester opening to remove accumulated sulfur in the headspace did not alter process performance once the microaerobic removal was restarted.

For more details, please use the following link:

http://wst.iwaponline.com/content/65/8/1368

4-Management of hydrogen sulfide in anaerobic digestion of enzyme pretreated marine macro-algae

Accumulation of large masses of algae on the beaches may happen seasonally as a result of green tides phenomenon. Long term resident of algal piles on the beaches would seriously cause inconveniences regarding recreational aspects of coastlines as well as negative environmental impacts. Occurrences of various insects in the piled masses and domination of anaerobic conditions leading to emissions of unpleasant smell (including hydrogen sulfide’s smell) are the most straight forward results of the phenomenon. Anaerobic digestion of algae

can be a beneficial process which not only is capable of solving the pollution problem mentioned above, but also has a potential to produce amounts of energy as biogas. On the way towards anaerobic digestion of algae, in a continuous implementation, crucial problems have to be faced and handled. Availability of algae year-round, collection, preparation, and finally its efficient digestion have significant challenges. In this study, it was tried to suggest an applicable method for pretreatment in addition to coping with hydrogen sulfide generation and its impact on digestion processes at thermophilic (55°C) and mesophilic (35°C) temperatures.

Enzymatic pretreatment of algae – by means of cellulose degrading enzyme – was evaluated through lab-scale and pilot-scale experiments. The degradation efficiency of the enzyme depended on the initial physical quality of the algae. The fresh algae collected in February 2012 was found to be resistant against enzymatic attacks, whereas already degraded algae, collected in April 2012, was further degraded by the enzyme resulting an efficiency of 40% regarding the solids content. Lab-scale batch anaerobic digestion experiments showed comparatively low methane potential for the pretreated algae at both mesophilic and thermophilic temperatures. However, the raw algae (cut into small pieces) were found to be hardly hydrolysable. The methane potential of raw algae in thermophilic and mesophilic digestion was about 17 NmL/gVS and − 36 NmL/gVS (negative value) respectively. Presence of inhibitory agent(s) was obvious at both temperatures.

Very fast growth of sulfate-reducing bacteria was noticed in the continuous digestion, so that in less than 20 days, hydrogen sulfide concentrations over 10000 ppm were observed in both meso- and thermophilic reactors. Inhibition of methanogenesis in the thermophilic reactor occurred at unionized dissolved sulfide concentration of about 22 mg/L (10000 ppm in the biogas) while it was mainly non-SRB acetogens that were inhibited in the mesophilic reactor at unionized sulfide concentrations as high as 50 mg/L (17000 ppm in the biogas). This shows that thermophilic digestion is more prone to be inhibited at high sulfide concentrations regarding methanogenesis. Micro-aeration was found to be more efficient in the thermophilic reactor while its effect on the mesophilic process was negligible. Addition of iron (III) chloride substantially decreased the sulfide level in the headspace of the reactors but the amount of sulfide reduction in the biogas (in the gas collector) was relatively lower. This could be accounted for different reaction rates of sulfidogenesis and sulfide oxidation. In this case high amounts of sulfide are generated and consequently transported into the gas collector while the overpressure in the headspace of the reactors does not allow backflow of hydrogen sulfide to the reactor for further oxidation, meanwhile major portion of the sulfide in the reactor can be oxidized.

For more details, please use the following link:

http://www.chemeng.lth.se/exjobbR/E654.pdf

Hoping this will be helpful,

Rafik

The most common methods for H2S reduction are biological desulfurization (with air / O2), chemicals (Iron Chloride is not so expensive, about 0,22-0,3 €/kg) and active carbon filter.

Operating in this way you can have low concentration of H2S (0-10 ppm) in your final biogas.

There are a lot of biogas plants with pig manure and protein rich substrates in table feeding and they solve H2S problem in this way.

Use of FeO or Fe2O3 could scrub S as FeSO4. Iron oxide is available in pellet form and is recoverable as well. So using this will be a one time cost actually. 

The manipulation of pH can be critical in determining whether sulfides found in one's slurry are expressed as H2S or not. This has been known for better than a half century, but for some reason is not often mentioned. Consider the linked chart, found in various publications, tracing back (as far as I can discover) to "Anaerobic Treatment in Pulp- and Paper Waste Management: A Review" (linked to this web site as shown below),

David William House, author, The Complete Biogas Handbook, www.completebiogas.com

Article Anaerobic treatment in pulp- and paper-mill waste management: A review

Thanks - I will be looking more closely at the use of ORP sensors with O2 sensor.  Also injection of pure O2 with thiobaccillus and I thank you for your valuable comments.  The manipulation of pH is really valuable in high sulphur slurries. I lean towards the idea that HS-  , which has the same toxicity as CN (Cyanide) is the ion that does the damage.  Therefore lowering pH is useful if the next stage is biological.  Thanks again.