Effect of Ecuadorian natural zeolite on the performance of anaerobic digestion of swine waste in semicontinuous regime

This study explores the potential of zeolite as an amendment to mitigate ammonium inhibition in the anaerobic digestion of swine waste. Two 50 L reactors, one with and one without zeolite amendment were operated at an OLR of 3.0 g VS L-1d-1 for 130 days, and fed with swine waste from a full-scale pig farm. Under these conditions, zeolite doses of 4 g L-1 allowed total ammonia nitrogen (TAN) concentrations to be kept below 1000 mgNH3-N L-1. The zeolite-amended reactor not only showed an average increase of 8% in methane production under stable conditions but also exhibited 34% reduction in H2S concentrations in the biogas, compared to the reactor without zeolite. The community of archaea originating from the inoculum was conserved in the reactor with zeolite amendment, particularly the acetoclastic methanogens of the genus Methanosaeta. On the other hand, in the reactor without zeolite addition, the microbial community went from being dominated by the acetoclastic methanogen Methanosaeta to having a high relative abundance of hydrogenotrophic methanogens. The zeolite addition also favoured the reactor stability, prevented foaming, and produced an enriched natural zeolite with N, P and K. However, additional studies on the potential of enriched zeolite as a fertilizer are required, which could make the use of zeolite in Anaerobic Digestion of swine waste not only energetically favourable but also economically feasible.

Natural Ecuadorian zeolite: An effective ammonia adsorbent to enhance methane production from swine waste

Anaerobic digestion (AD) of swine waste allows obtaining renewable energy, biofertilizer and the reduction of environmental impacts. However, the low C:N ratio of pig manure generates high concentrations of ammonia nitrogen in the digestion process, reducing methane production. Zeolite is an effective ammonia adsorbent; thus, in this research the ammonia adsorption capacity of natural Ecuadorian zeolite was studied under different operating conditions. Subsequently, its effect on methane production from swine waste was evaluated using three doses of zeolite, 1.0, 4.0 and 8.0 g, in 1 L batch bioreactors. The results showed that the Ecuadorian natural zeolite has an adsorption capacity of around 19 mgNH₃-N gZ^-1 when using ammonium chloride solution and, an adsorption capacity between 37 and 65 mgNH₃-N gZ^-1 using swine waste. On the other hand, the addition of zeolite had a significant effect on methane production (p < 0.01). The zeolite doses that provided the highest methane production were 4.0 and 8.0 g L^-1, which led to values of 0.375 and 0.365 Nm³CH₄ kgVS^-1, compared to the values of 0.350 and 0.343 Nm³CH₄ kgVS^-1 that were obtained for the treatments without addition of zeolite and using a dose of 1.0 g L^-1, respectively. Addition of natural Ecuadorian zeolite meant not only a significant increase on methane production in the AD of swine waste, but also a better quality of the biogas with higher percentages of methane and lower concentrations of H₂S.

Recovering biomethane and nutrients from anaerobic digestion of water hyacinth (Eichhornia crassipes) and its co-digestion with fruit and vegetable waste

The potential to recover bioenergy from anaerobic digestion of water hyacinth (WH) and from its co-digestion with fruit and vegetable waste (FVW) was investigated. Initially, biogas and methane production were studied using the biochemical methane potential (BMP) test at 2 g volatile solids (VS) L(-1) of substrate concentration, both in the digestion of WH alone and in its co-digestion with FVW (WH-FVW ratio of 70:30). Subsequently, the biogas production was optimized in terms of total solids (TS) concentration, testing 4 and 6% of TS. The BMP test showed a biogas yield of 0.114 m(3) biogas kg(-1) VSadded for WH alone. On the other hand, the biogas potential from the WH-FVW co-digestion was 0.141 m(3) biogas kg(-1) VSadded, showing an increase of 23% compared to that of WH alone. Maximum biogas production of 0.230 m(3) biogas kg(-1) VSadded was obtained at 4% of TS in the co-digestion of WH-FVW. Using semi-continuously stirred tank reactors, 1.3 m(3) biogas yield kg(-1) VSadded was produced using an organic loading rate of 2 kg VS m(-3) d(-1) and hydraulic retention time of 15 days. It was also found that a WH-FVW ratio of 80:20 improved the process in terms of pH stability. Additionally, it was found that nitrogen can be recovered in the liquid effluent with a potential for use as a liquid fertilizer.

Production of volatile fatty acids from wastewater screenings using a leach-bed reactor

Screenings recovered from the inlet works of wastewater treatment plants were digested without pre-treatment or dilution using a lab-scale, leach-bed reactor. Variations in recirculation ratio of the leachate of 4 and 8 l/lreactor/day and pH values of 5 and 6 were evaluated in order to determine the optimal operating conditions for maximum total volatile fatty acids (VFA) production. By increasing the recirculation ratio of the leachate from 4 to 8 l/lreactor/day it was possible to increase VFA production (11%) and soluble COD (17%) and thus generate up to 264 g VFA/kg-dry screenings. These VFA were predominantly acetic acid with some propionic and butyric acid. The optimum pH for VFA production was 6.0, when the methanogenic phase was inhibited. Below pH 5.0, acid-producing fermentation was inhibited and some alcohols were produced. Ammonia release during the hydrolysis of screenings provided adequate alkalinity; consequently, a digestion process without pH adjustment could be recommended. The leach-bed reactor was able to achieve rapid rates of screenings degradation with the production of valuable end-products that will reduce the carbon footprint associated with current screenings disposal techniques.

Methane production and hydrolysis kinetics in the anaerobic degradation of wastewater screenings

Anaerobic biodegradability and hydrolysis rates of wastewater screenings were determined using the biochemical methane potential test at 37 °C. The extent and rate of screenings conversion to methane of this complex and particulate substrate were investigated and since two stages of hydrolysis were identified, corresponding to the different types of materials in screenings, a linear and non-linear model was used. No accumulation of intermediary products was observed and so it was possible to use the methane production rate and a linear model to estimate the hydrolysis rate in the first phase of hydrolysis. The measured values of 0.061-0.127 d(-1) are in the range reported for other comparable organic wastes. It was also observed that the inoculum-to-substrate ratio has a large impact on methane production rate of screenings. The difference in biodegradation rates from the materials in screenings and the overall hydrolysis could be represented by the modified Gompertz non-linear model which was able to describe the methane production rate of screenings with a high confidence. Screenings were found to have 52% biodegradability on average and this shows the potential for volatile solids destruction. A two-stage process with an improved hydrolysis rate is proposed to ensure that the full potential of the material is exploited.