Please contact me directly for assistance or more information about the bulletins at rolf.skea@resenviro.com

The descriptions below are process & technology Information Communication output, designed to inform personnel with responsibilities operating, optimizing and managing wastewater treatment discharge. The bulletins are short communications authored to address known chemistry and bio-chemistry aspects that impacts the environment.

To establish a well-balanced activated sludge system for nitrous oxide and nitrate control, a suitably large anoxic zone is needed. Nitrous oxide may be generated when ammonium is enzymatically oxidized to nitrate under low DO conditions. When heterotrophic bacteria reduce nitrogen back from nitrate, four different enzymes are involved, leading to an even greater chance of nitrous generation under stressed conditions. The activated sludge system shown here, has a buffer, dilution tank addition, integrated into the process following primary settlement.

Recirculated nitrate from the return sludge, and recirculated final effluent, are mixed with settled crude with high concentration of short chain fatty acids. The anoxia in the tank allows for favorably conditions for rapid denitrification. At the same time, the ammonia concentration can be managed for slow release into the reactor under high load conditions with the use of dilution water from the final effluent.

Let’s consider a conventional air activated sludge system, with an additional balance tank and effluent re-circulation system. In the sewerage collection system, before wastewater arrive in primary settlement, conditions favour anaerobic fermentation. Emissions from the PST are therefore products of hydrolyses and acidification. Carbon dioxide and reduced sulphur gas, Hydrogen Sulfide, and reduced organic sulphur vapours, mercaptans, are the emissions of concern. The organic sulphur mercaptans are responsible for the strong odours around PST systems and sludge tanks.

In the aeration reactor of secondary treatment, biodegradable nutrients, free fatty acids, and ammonium are oxidised by the concentrated bio-culture to form carbon dioxide and nitrate respectively. Both reaction pathways involve dissolved oxygen as electron acceptors for the generation of energy for the organisms involved. Nitrification is completed by two types of organisms and the reaction intermediates catalysed by enzyme reactions may lead to the generation of excess nitrous oxide under less favourable conditions. Dissolved oxygen levels are therefore key to managing the biochemical reactions in secondary treatment, and their emissions.

The process emissions from the aeration reactor are mostly carbon dioxide to air, and carbon dioxide as dissolved greenhouse gas to the effluent water. Notable nitrous oxide greenhouse gas emissions do occur from the main reactor, but the nitrous emissions are very low, less than 1 percent of total Greenhouse Gas emissions in well managed activated sludge systems. The highest impact greenhouse gas emission comes from dissolved carbon dioxide in the final effluent, with its double impact of direct consumption of free alkalinity and changing the equilibrium of atmospheric carbon dioxide solubility. In simple terms, the dissolved CO2 two causes ocean acidification, and it prevents atmospheric c o two dissolution into the ocean CO2 sink.

Strong odours and toxic gases from wastewater treatment processes are not only serious health hazards, but may also lead to nuisance complaints from local communities. Hydrogen sulphide is formed through the reduction of the sulphate, but the strong smells are from reaction products of the SH2 to form various mercaptan reaction products. 

Food and, therefore, sewage contain significant levels of sulphate, which serve as a sulphate-oxygen bond source for sulphate-reducing bacteria in and around anaerobic processes and sludge tanks. The bacteria reduce sulphur, forming hydrogen sulphide (H2S), a potent nucleophile that can attack other organic compounds in the sludge via nucleophilic substitution. 

It is the same biochemical reactivity that imparts SH2 toxicity. SH2 is a very toxic gas that can only be detected at low concentrations by smell.  At around 100 ppb (0.005 – 0.3 ppm), the nose's sensitivity is numbed, and the gas becomes extremely dangerous to breathe. At 700 – 1000 ppm, exposure to SH2 may be lethal. Chronic exposure may lead to respiratory, neurological or ocular health complications. 

In many ways, the mercaptan smells around waste sludge tanks and sludge discharge points are welcome reminders that there may be high concentrations of SH2 gas in the immediately breathable atmosphere. Health and safety concerns for personnel working around these processes or in temporarily erected office space directly next to a sludge handling operation are well warranted. 

To form the organic mercaptans that cause foul odours in wastewater processes and sludge, nucleophilic substitution occurs with organic molecules such as volatile fatty acids (VFAs), alcohols, and alkanes. Molecules like propionate, oxaloacetate and butyrate are very common metabolites present in the AD and AS process. 

Common mercaptans found in foods such as garlic and onions are known for their strong odour and pungent taste. These mercaptans are regularly formed in sludges that become anaerobic and reductive.