Sulfur transformations during two-stage anaerobic digestion and intermediate thermal hydrolysis.

The formation of hydrogen sulfide (H2S) during anaerobic digestion (AD) imposes constraints on the valorisation of biogas. So far, inorganic sulfur compounds -mainly sulfate - have been considered as the main contributors to H2S formation, while the contribution of organic sulfur compounds is mostly neglected. This study investigates the fate of organic and inorganic sulfur compounds during two-stage anaerobic digestion with intermediate thermal hydrolysis for treatment of primary and secondary sludge in a WWTP treating domestic wastewater. The results of a seven-week monitoring campaign showed an overall decrease of organic sulfur compounds in both stages of anaerobic digestion. Further fractionation of organic sulfur revealed a high conversion of the particulate organic fraction during the first digestion stage and of the soluble organic fraction during the second digestion stage. The decrease of soluble organic sulfur during the second digestion stage was attributed to the solubilisation and hydrolysis of sulfur-containing organic compounds during thermal hydrolysis. In both digestion stages, more organic sulfur was taken up than particulate inorganic sulfur (metal sulfide) was produced, indicating the formation of other reduced sulfur forms (e.g. H2S). Further batch experiments confirmed the role of organic sulfur uptake in the formation of H2S during anaerobic digestion as sulfate reduction only partly explained the total sulfide formed (H2S in biogas and precipitated FeS). Overall, the conversion of organic sulfur was demonstrated to play a major role in H2S formation (and thus the biogas quality), especially in case of thermal hydrolysis pretreatment.

Plant-wide investigation of sulfur flows in a water resource recovery facility (WRRF).

Even though sulfur compounds and their transformations may strongly affect wastewater treatment processes, their importance in water resource recovery facilities (WRRF) operation remains quite unexplored, notably when it comes to full-scale and plant-wide characterization. This contribution presents a first-of-a-kind, plant-wide quantification of total sulfur mass flows for all water and sludge streams in a full-scale WRRF. Because of its important impact on (post-treatment) process operation, the gaseous emission of sulfur as hydrogen sulfide (H2S) was also included, thus enabling a comprehensive evaluation of sulfur flows. Data availability and quality were optimized by experimental design and data reconciliation, which were applied for the first time to total sulfur flows. Total sulfur flows were successfully balanced over individual process treatment units as well as the plant-wide system with only minor variation to their original values, confirming that total sulfur is a conservative quantity. The two-stage anaerobic digestion with intermediate thermal hydrolysis led to a decreased sulfur content of dewatered sludge (by 36%). Higher (gaseous) H2S emissions were observed in the second-stage digester (42% of total emission) than in the first one, suggesting an impact of thermal treatment on the production of H2S. While the majority of sulfur mass flow from the influent left the plant through the treated effluent (> 95%), the sulfur discharge through dewatered sludge and gaseous emissions are critical. The latter are indeed responsible for odour nuisance, lower biogas quality, SO2 emissions upon sludge combustion and corrosion effects.

Impact on crops, plants and soils of metal trace elements transfer and flux, after spreading of fertilizers and biosolids.

In France, the yearly production of sludge from wastewater treatment plants is 900,000 metric tons dry matter and 60% of this is reused for land application. Today, the sustainability of this pathway is open to question. Among the different arguments cited are the levels of metal trace elements and the risks of accumulation in soils. With the ultimate aim of agronomic sludge recycling, the transfer of metal trace elements has been studied using vegetation containers planted with rye-grass under controlled conditions of temperature and humidity. Samples of a domestic sludge, an industrial sludge and a fertilizer have been mixed with the soil. By monitoring the growth of the rye-grass, we have been able to observe that the addition of sludge increases production of plant matter. It appears that the roots absorb higher quantities of metal trace elements and form a barrier to their transfer to the above ground parts of the rye-grass. For the group of metal trace elements studied, no significant differences have been observed between the rye-grass grown on soil alone and that on soils amended with fertilizer or urban sludge. For the majority of the vegetation containers studied, there has been no significant modification in the soil metal distribution over time, as a result of the addition of urban sludge, and no significant difference between fertilizers and sludges.

The influence of biosolids treatment files on the mobility of metal trace elements.

The production of sludge in France is estimated to be about 900,000 metric tons dry matter per year and 60% of this is recycled onto agricultural land. At present, the long term future of this procedure is open to question and among the different arguments being put forward are the levels of metallic trace elements and the risk of accumulation in soils. This study presents the behaviour of metallic trace elements in sludges from three different treatment procedures: thickened liquid sludges, dewatered sludges and dried sludges. These biosolids are mixed with a clay soil and then placed in a temperature and humidity controlled glasshouse. Several containers are seeded with ryegrass and compared with controls. For the three harvests, covering all the amendments studied (including non-amended soil), the differences are not really representative. Absorption by the ryegrass is low in all cases. For the cadmium, the chromium, the nickel and the lead, the roots are 5 to 10 times more concentrated than the leaves. The majority of these elements stay absorbed in the roots, regardless of the amendment used. The addition of the sludges has considerably reduced the uptake of water in ryegrass throughout its growth cycle. Quite apart from their fertilizing qualities, wastewater treatment plant sludges could offer important implications for irrigation.

Comparative evaluation of the potential noxiousness in domestic sludge used in agriculture and in commercial fertilizers.

The noxiousness of actual sludge collected in eight water treatment plants around the city of Toulouse, France, was evaluated using a biological test based on the growth rate of cultured human cells. Results were compared with those obtained from 18 fertilizers and culture supports that are commercially available in gardening shops. Surprisingly, it was found that sludge extracts, at low concentrations (below 5 g of dry material/liter), were improving the cell growth rate, which suggests the presence of useful oligoelements. At higher concentrations, a noxious effect, expressed as inhibition of cell growth, was observed. However, this negative effect was of the same order of magnitude as that obtained, under the same experimental conditions, with commercial garden fertilizers which are available and used without any restriction. It is concluded that discarding the sludge, after submission to the biological test, in controlled amount as an agricultural fertilizer should not be hazardous to the environment.