Comparison of Pre-treatment Technologies to Improve Sewage Sludge Biomethanization.

This research study evaluates various pre-treatments to improve sewage sludge solubilization prior to treatment by mesophilic anaerobic digestion. Microwave, thermal, and sonication pre-treatments were compared as these pre-treatments are the most commonly used for this purpose. The solubilization of sewage sludge was evaluated through the variation in soluble total organic carbon (sTOC, mg/L) and soluble total nitrogen (sTN, mg/L). Thermal and microwave pre-treatments increased sTOC/VS by 19.2% and 83.4% (VS, total volatile solids), respectively, after applying lower specific energy through (20 kJ/g TS, approximately) (TS, total solids) unlike the sonication pre-treatment, which required 136 kJ/g TS. Although sTN content did not increase significantly with the pre-treatments with respect to sTOC, both showed proportional trends. Sonication pre-treatments allowed the highest increase in volatile fatty acids (VFA) with respect to the raw sewage sludge (15% ∆VFA/sTOC). Methane production with and without pre-treatment was also evaluated. Methane production increased by 95% after applying sonication pre-treatment compared to the methane production of raw sewage sludge. Thermal and microwave pre-treatments entailed lower improvements (29% and 20%, respectively). Economically, thermal pre-treatments were the most viable alternative at real scale. Graphical abstract.

Wastewater nutrient recovery using twin-layer microalgae technology for biofertilizer production.

This study evaluates the feasibility of advanced biofilm microalgae cultivation in a twin layer (TL) system for nutrient removal (N and P) as the tertiary treatment in small wastewater treatment plants (WWTPs) located in sensitive areas. Furthermore, the potential valorisation of microalgae biomass as a component of bio-based fertilizers is assessed. Scenedesmus sp. was chosen among 33 microalgae strains for inoculation of TL due to its high growth rate and its nutrient uptake capacity. The tests carried out in the prototype were markedly efficient for total soluble and ammoniacal nitrogen removal (up to 66 and 94%, respectively). In terms of potential valorisation of microalgae, the nutrient content was 5.5% N (over 40% protein), 8.8% P2O5 and 1.5% K2O, high enzymatic activity, very low levels of heavy metals and no detectable pathogen presence. However, in the formulation of solid-state bio-based fertilizers, the microalgae proportions in blends of over 2% of microalgae led to negative effects on ryegrass (Lolium perenne L. ssp.) and barley (Hordeum vulgare ssp.). The obtained results demonstrate that TL represents a promising technology, which allows efficient tertiary treatment of urban wastewater and the production of high-quality bio-based fertilizer.

Bacteria, archae, fungi and viruses: it takes a community to eliminate waste.

Importance of the microbiota communities for improving the efficiency in Waste Water Treatment Plants.

Mixture optimization of anaerobic co-digestion of tomato and cucumber waste.

Greenhouse cultivation has significantly increased the production of vegetables and reduced dependence on environmental conditions. In Mediterranean areas, vegetable crops are nowadays one of the most important sources of organic waste generation. Anaerobic digestion is among the methods used to treat this type of biodegradable waste. However, the selection of the organic wastes to be subjected to this microbial treatment is a crucial aspect due to seasonality and simultaneity of the original crops. In this sense, as waste does not have frequently the proper nutrient balance, co-digestion with other substrates generated in the same geographical area is recommended to ensure the correct development of the process. This work studies the mesophilic co-digestion of tomato waste (TW) and cucumber waste (CW), which are common greenhouse wastes that do not contain an adequate ratio among nutrients (C/N/P) and are generated simultaneously. The influence of the percentage of both wastes in the mixture on the overall performance of the process was evaluated. The combination of TW and CW was found to be feasible in terms of stability, biodegradability and methane yield, which reached a value of 292 mLSTP CH4/g VS (STP: standard temperature and pressure, 0°C and 760 mmHg) for the percentage of tomato in the mixture, which is considered optimal at 55-75%. The most suitable organic load rate was determined for a percentage of 65% of TW, reaching a value of 1562 kg(waste) m(-3) month(-1).

Evaluation of the improvement of sonication pre-treatment in the anaerobic digestion of sewage sludge.

Sewage sludge is a polluting and hazardous waste generated in wastewater treatment plants with severe management problems. The high content in heavy metal, pathogens and micropolluting compounds limit the implementation of the available management methods. Anaerobic digestion could be an interesting treatment method, but must be improved since the biomethanisation of sewage sludge entails low biodegradability and low methane production. A sonication pre-treatment at lab scale is proposed to increase the organic matter solubilisation of sewage sludge and enhance the biomethanisation yield. Sonication time was optimised by analysing the physicochemical characteristics of sewage sludge (both total and soluble fraction) at different pre-treatment times. The pre-treatment time was fixed at 45 min under the study conditions given that the solubilisation of organic matter did not increase significantly at lower sonication times, whereas the concentration of total nitrogen increased markedly at higher times. The volatile fatty acids generation rate was also evaluated for the pre-treatment conditions. The anaerobic digestion of untreated and pre-treated sewage sludge was subsequently compared and promising results were obtained for loads of 1.0 g VS/L (VS, total volatile solids). The methane yield coefficient increased from 88 to 172 mLSTP/g VS (STP, 0 °C, 1 atm) after the pre-treatment, while biodegradability was found to be around 81% (in VS). Moreover, the allowed organic loading rate and methane production rate observed for the sewage sludge reached values of up to 4.1 kg VS/m(3)·d and 1270 LSTP/m(3)·d, respectively.

Anaerobic digestion of wastewater derived from the pressing of orange peel generated in orange juice production.

A study of the anaerobic digestion of wastewater from the pressing of orange peel generated in orange juice production was carried out in a laboratory-scale completely stirred tank reactor at mesophilic temperature (37 degrees C). Prior to anaerobic treatment the raw wastewater was subjected to physicochemical treatment using aluminum sulfate as a flocculant and to pH reduction using a solution of sulfuric acid. The reactor was batch fed at COD loads of 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, and 5.0 g of COD. The process was very stable for all of the loads studied, with mean pH and alkalinity values of 7.5 and 3220 mg of CaCO3/L, respectively. The anaerobic digestion of this substrate was found to follow a first-order kinetic model, from which the specific rate constants for methane production, K(G), were determined. The K(G) values decreased considerably from 0.0672 to 0.0078 L/(g h) when the COD load increased from 1.5 to 5.0 g of COD, indicating an inhibition phenomenon in the system studied. The proposed model predicted the behavior of the reactor very accurately, showing deviations of <5% between the experimental and theoretical values of methane production. The methane yield coefficient was found to be 295 mL of CH4 STP/g of COD removed, whereas the mean biodegradability of the substrate (TOC) was 88.2%. A first-order kinetic model for substrate (TOC) consumption allowed determination of the specific rate constants for substrate uptake, K(C), which also decreased with increasing loading, confirming the above-mentioned inhibition process. Finally, the evolution of the individual volatile fatty acid concentrations (acetic, C2; propionic, C3; butyric, C4; isobutyric, iC4; valeric, C5; isovaleric, iC5; and caproic, C6) with digestion time for all loads used was also studied. The main acids generated were acetic and propionic for all loads studied, facilitating the conversion into methane.