Development of statistical predictive models for estimating the methane yield of Italian municipal sludges from chemical composition: a preliminary study.

The biochemical methane potential (BMP) of primary and biological sludge varies in a wide range, mostly depending on location, sewer characteristics, wastewater treatment plant design and operating conditions. BMP tests are useful to verify the performance of a full scale digester, but they are not yet a common procedure in the operation of most Italian facilities because of cost and test duration. Changes in the composition of sewage sludge can lead to a high variation of biogas production. Aimed at developing BMP predictive models based on low cost and fast analyses, this study investigated the chemical composition of 20 sludge samples by means of principal component and multiple linear regression analyses. Three preliminary predictive models were developed based on soluble organic nitrogen, volatile solids, carbohydrates, proteins, lipids and an operational parameter, the sludge retention time: the explained variance and the standard errors of prediction of BMP are in the range 77-81% and 21-34 NmLCH4·gVS -1, respectively. Models were evaluated on five additional samples: errors ranged 2-15% for four samples and about 54% for one sample, collected from a peculiar facility. Further data and variables describing the operation mode of the waterline would certainly improve the reliability and robustness of the models.

Hydrogenotrophic biogas upgrading integrated into WWTPs: enrichment strategy.

Within the European circular economy roadmap, it is important for wastewater treatment plant (WWTPs) to recover energy and become energy-neutral or -positive. In the last few years, it has become increasingly interesting to boost energy recovery through the biogas upgrading. The aim of this work is to study a rapid hydrogenotrophic methanogenic culture enrichment strategy capable of limiting the organic degradation unbalance and allowing a fast start-up phase of the in situ biogas upgrading reactors, at pilot- or full-scale. The approach was tested with two, plus one control, laboratory-scale continuous stirred tank reactors filled with anaerobic sludge collected from a full-scale WWTP. The experimentation lasted 50 days and was divided into five phases: the anaerobic digestion start-up followed by four H2 injection phases (H2/CO2 ranging from 1:1 to 4:1 on molar basis). Despite a temporary slight increase in the total concentration of volatile fatty acids during phase II (2.56 gCH3COOH·L-1), and in phase III a mild pH increase (anyway, below 7.4) indicating the expected CO2 depletion, the strategy proposed was effective. In the last phase, in the biogas a methane content of about 80% was achieved, thus suggesting that the use of H2/CO2 above the stoichiometric value could further improve the biological biogas upgrading.

MBR fouling control and permeate quality enhancement by polyaluminium chloride dosage: a case study.

Scope of this study was to evaluate the effectiveness of a metal salt (polyaluminium chloride, PACl) dosage into a pilot-scale MBR (membrane bioreactor) in terms of fouling control and permeate quality enhancement, especially with reference to specific textile macro-pollutants (dyes and surfactants). The pilot plant was fed with a mixed domestic-textile wastewater (textile wastewater accounted for 65% of total flow and for 70% of total chemical oxygen demand, COD, load) and operated for 7.5 months without flux enhancers (step 1) and 3 months with the addition of PACl (step 2). The optimum dose was defined performing a jar-test campaign between step 1 and step 2 (12.5 mg gMLSS(-1) that corresponds to 0.4 g d(-1)). The addition of PACl resulted in a significant decrease of the filtration resistance due to cake layer formation (R(c), -65.4%) and of the irreversible fouling rate, evaluated as the average variation per unit time of the filtration resistance due to foulants adsorption on membrane pore wall (FR, -45.3%). As for permeate quality, removal rates related to total phosphorus and textile macro-parameters such as colour and anionic surfactants, increased by +64, +16 and +7%, respectively. No significant effect was observed on COD, non-ionic surfactants and nitrogen compounds removal.

Sodium hydroxide pretreatment of ensiled sorghum forage and wheat straw to increase methane production.

The aim of this study was to determine the effect of sodium hydroxide pretreatment on the chemical composition and the methane production of ensiled sorghum forage and wheat straw. NaOH pretreatment was conducted in closed bottles, at 40 °C for 24 h. Samples were soaked in a NaOH solution at different dosages (expressed in terms of total solids (TS) content) of 1 and 10% gNaOH/gTS, with a TS concentration of 160 gTS/L. At the highest NaOH dosage the reduction of cellulose, hemicelluloses and lignin was 31, 66 and 44%, and 13, 45 and 3% for sorghum and wheat straw, respectively. The concentration of soluble chemical oxygen demand (CODs) in the liquid phase after the pretreatment was also improved both for wheat straw and sorghum (up to 24 and 33%, respectively). Total sugars content increased up to five times at 10% gNaOH/gTS with respect to control samples, suggesting that NaOH pretreatment improves the hydrolysis of cellulose and hemicelluloses. The Biochemical Methane Potential (BMP) tests showed that the NaOH pretreatment favoured the anaerobic degradability of both substrates. At 1 and 10% NaOH dosages, the methane production increased from 14 to 31% for ensiled sorghum forage and from 17 to 47% for wheat straw. The first order kinetic constant increased up to 65% for sorghum and up to 163% for wheat straw.

Biochemical characterization and anaerobic degradability of flower wastes: Preliminary assessment and statistical interpretation towards energy recovery.

The use of ornamental flowers and plants is widespread in several regions of the world, but the management of flower (or floral) waste (FW), classified as herbaceous biomasses, is scarcely addressed in the literature. However, climate change, population growth and the depletion of resources are expected to push towards the development of FW management strategies, according to principles of flexibility and integration of technologies. This study focuses on the characterization of ten different varieties of flowering plants, of which the wastes are of concern in the Pistoia Province (Italy). The possibility of recovering energy by means of anaerobic digestion is also preliminarily investigated. The interpretation of data through Principal Component Analyses proved to be effective to orientate the selection of technological solutions. The three main parts of each plant variety were analysed separately, showing that the biochemical composition of stems is statistically different from that of leaves and flowers, thus suggesting the viability of adopting different strategies to optimize material (value-added products) recovery from FW. Conversely, regarding biogas generation and energy recovery, the methane yield (in the range 82-330 NmLCH4.gVS-1) is not significantly affected by the type of FW part, nor by the variety of flowering plant or by the use of pesticides during cultivation, whereas lower kinetics were observed for stems compared to leaves and flowers. In view of full-scale application, and depending on FW amounts locally produced, a careful evaluation is required, encompassing aspects of technical feasibility and economic expenses associated with FW parts separation.

Influence of thermal extraction of extracellular polymeric substances on cell integrity in activated sludge and membrane bioreactor samples.

The influence of the soluble microbial products (SMP) and extracellular polymeric substances (EPS) heating extraction method on cell viability was evaluated for each phase of the protocol using epifluorescence microscopy. In addition, the effect of different centrifugation conditions (2700 g at 24 degrees C; 12,000 g at 4 degrees C) was also tested. Sludge samples were collected from a conventional wastewater treatment and a membrane bioreactor (MBR) pilot plant fed in parallel. Results show that different centrifugation parameters do not induce cell membrane damaging. Heating significantly influences membrane integrity; for instance, 75 to 90% of initial viable cells are damaged during this phase, possibly leading to the predominance of protein compared to carbohydrate content. The protein content in EPS is 60 to 88 mg bovine serum albumin/ g volatile suspended solids (VSS); higher values observed in MBR sludge samples are probably attributable to the different characteristics of microbial flocs and process operating parameters. Carbohydrate concentrations are not significantly different regardless of applied procedure and sludge type, and are between 10.4 to 11.6 mg glucose/g VSS.

Adsorption and removal at low atrazine concentration in an MBR pilot plant.

Atrazine is a persistent organic pollutant and it has been widely used in agriculture and forestry in the world for more than fifty years. Atrazine shows ecotoxicity effects in aquatic ecosystems even at very low level concentrations with endocrine disruptor activity. Few studies were carried out on atrazine removal performances in drinking and waste-water by biological treatments, especially in membrane bio-reactors (MBRs). MBR technology might be more efficient than the conventional one in the removal of micro-pollutants. The fate of atrazine in wastewater treatment plants and its influence on the biomass activity was evaluated in this study. The experimental work was divided in three different phases: inhibition studies on different types of biomass (by means of microcalorimetry); adsorption studies on different sludges (conventional activated sludge (CAS) - and MBR) calculating adsorption isotherms and, finally, atrazine removal in an MBR pilot plant (simulating a treatment of atrazine and nitrate contaminated groundwater). The absence of significant inhibition was observed; higher atrazine adsorption on MBR sludge was detected for lower atrazine concentration (<50 µg L(-1)); the removal efficiency in the MBR pilot plant was lower than 25% but higher than the theoretical one (based on adsorption isotherms).

The fate of 14C-radiolabelled diclofenac and 4'-hydroxydiclofenac in membrane bioreactor treatment of wastewater.

This study aimed at enhancing knowledge on the fate of diclofenac (DF), together with its main human metabolite 4'-hydroxydiclofenac (4'OHDF), during wastewater treatment by using a laboratory-scale membrane bioreactor (MBR). The reactor was fed continuously with non-radiolabelled diclofenac for a one month period prior to a single pulse of a 14C-radiolabelled solution of DF and 4'OHDF. The solution spike contained approximately 25% 4'OHDF and 65% DF, which corresponds to the ratio observed in municipal sewage, as well as traces of two other metabolites. The radioactivity was monitored for a total of twelve days in the various output streams. The calculation of the complete mass balance in the system demonstrated that the major part of the radioactivity left the reactor with the permeate (88.7%), while 2.1% was recovered in the excess sludge. Negligible amounts were recovered in the off-gas traps and on the membranes. Chromatographic analyses of effluent samples, by means of HPLC-MS coupled in parallel to a radiodetector, displayed a different pattern than the one of the spiked solution. It showed the occurrence of three additional metabolites.

Microcalorimetric and manometric tests to assess anammox activity.

The present study compares two experimental methods to evaluate Anammox activity based on the assessment of (1) the N(2) production rate by a manometric device, as previously proposed, and (2) the heat production rate by a microcalorimeter. Two samples of Anammox suspended biomass were taken from a pilot-plant, and their specific Anammox activity measured by both techniques. Both methods were successfully applied. As for calorimetric tests, they were performed for the first time on Anammox enriched sludge samples. Comparisons between the specific Anammox activities estimated by manometry and calorimetry and between expected (from the reaction enthalpy) and measured heat productions were performed. Promising results were obtained.

The use of microcalorimetry to compare the biological activity of a CAS and a MBR sludge-application to pharmaceutical active compounds.

Micropollutants removal, such as pharmaceutical substances, during wastewater treatment processes is becoming a greater issue everyday. In order to optimize it, their biodegradation processes have to be better understood. So far, microcalorimetry has been used worldwide to investigate chemical reactions. For few years now it has also been developed to model and control biological processes. In the case of micropollutants, respirometry is, most of the time, not precise enough to determine biodegradation. That is why, microcalorimetric experiments have been set up. For this purpose, a 2 L Bio-RC1 (Mettler-Toledo) has been modified to reach a resolution of 5-10 mW.L(-1).In this study, the biodegradation rate of standard substrates (Ethanol and Ammonia) by an activated sludge from a full-scale CAS and one from a MBR pilot plant operating in parallel have been compared by means of microcalorimetry. Then few trials to determine the biodegradation of selected pharmaceutical substances, as well as the eventual inhibition induced by them have been made. The first results did not exhibit any biological activity. However, they have displayed inhibition for both studied substances.

A simple method to evaluate the short-term biogas yield in anaerobic codigestion of WAS and organic wastes.

The present study was aimed at setting and applying a procedure to measure the anaerobic degradability of different organic substrates by short-term tests (2-7 days) carried out at lab-scale with a low food to biomass (F/M) ratio. All tests were carried out using an acclimated sludge taken from a pilot-plant anaerobic digester (200 L). Trials were performed with a manometric system. The experimental reliability of the device in measuring the anaerobic degradability was assessed by several preliminary tests carried out using acetate and glucose as reference substrates. The average conversion to methane was 99% for acetate and of 83% for glucose. The results of tests in triplicate showed the high repeatability of the method with an average coefficient of variation lower than 2%. Then, the lab-scale procedure was applied to study the short-term anaerobic degradability of complex organic substrates: thickened waste activated sludge, two kinds of organic fraction of municipal solid waste (a kitchen waste and a fruit and vegetable waste collected at the wholesale market of Florence), olive mill wastewater and freshly harvested grass. Results indicated that organic fraction of municipal solid waste, olive mill wastewater and grass were characterized by a much higher anaerobic degradability if compared to the thickened activated sludge, well in agreement with literature data.

Inhibition on anammox bacteria upon exposure to digestates from biogas plants treating the organic fraction of municipal solid waste and the role of conductivity.

The aim of this research was to evaluate the applicability of the anammox process for removing nitrogen from the supernatant originating from the anaerobic digestion of the organic fraction of municipal solid waste (OFMSW). The short term inhibitory potential of this concentrated wastewater was evaluated by means of batch tests in terms of maximum specific anammox activity reduction. A total of 20 real wastewater samples were tested originating from 4 different full scale anaerobic digestion plants treating OFMSW. Activity reduction between 73% and 89% was observed in the presence of undiluted real wastewaters. The specific activity remained stable for 6-7days after the initial reduction, thus suggesting its treatability even without dilution. The inhibitory effect of both the real and synthetic saline media tested could be modelled as a function of conductivity. IC50 of 6.1mS/cm was obtained for exposure to the tested liquid fraction of biowaste digestate.

Effect of the concentration of essential oil on orange peel waste biomethanization: Preliminary batch results.

The cultivation of orange (Citrus×sinensis) and its transformation is a major industry in many countries in the world, it leads to the production of about 25-30Mt of orange peel waste (OPW) per year. Until now many options have been proposed for the management of OPW but although they are technically feasible, in many cases their economic/environmental sustainability is questionable. This paper analyse at lab scale the possibility of using OPW as a substrate for anaerobic digestion. Specific objectives are testing the possible codigestion with municipal biowaste, verifying the effect on methane production of increasingly high concentration of orange essential oil (EO, that is well known to have antioxidant properties that can slower or either inhibit biomass activity) and obtaining information on the behaviour of d-limonene, the main EO component, during anaerobic digestion. The results indicate that OPW can produce up to about 370LnCH4/kgVS in mesophilic conditions and up to about 300LnCH4/kgVS in thermophilic conditions. The presence of increasingly high concentrations of EO temporary inhibits methanogenesis, but according to the results of batch tests, methane production restarts while d-limonene is partially degraded through a pathway that requires its conversion into p-cymene as the main intermediate.

Autotrophic nitrogen removal by a two-step SBR process applied to mixed agro-digestate.

The aim of this research was to evaluate the applicability of partial-nitritation/anammox processes for biological N removal from a centrifuge supernatant coming from a full scale anaerobic digester fed on a mixture of piggery manure, poultry manure, and agro-wastes. Stable partial nitritation was achieved at pilot-scale (650L SBR), obtaining a suitable influent for the anammox lab-scale SBR reactor (3L). The anammox lab scale reactor was fed with increasing fractions of the partial nitritation effluent, blended with synthetic wastewater. In the last 100days no dilution was used. The nitrogen loading rate applied to the anammox reactor was 0.5-0.6gNL(-)(1)d(-)(1) and the average nitrogen removal was 91±10%. During the first days of operation with undiluted supernatant, the maximum anammox activity in the SBR decreased, but recovered afterwards, suggesting the ability of the anammox biomass to acclimate to the wastewater. N2O emissions in both reactors were also measured.

Some economic considerations on wastewater reclamation for irrigation, with reference to the Italian situation.

The purpose of this work is to check construction and operation costs for simplified post-treatment trains able to produce water displaying a quality suitable for different agricultural uses. Having in mind the quality of most of surface waters in Italy, the main parameters to control for a general purpose reclaimed water supply are microbial quality and TSS content. Therefore a contact filtration followed by disinfection is to be considered the minimum option, while the use of polishing ponds can be strictly limited in many Italian regions due to the area requirements and to the need to provide a full lining of bottom and levees. Some data referring to surface waters quality and to the Italian epidemiological situation are given. On the basis of the current Italian situation, the construction and operation cost increases due to a polishing section, downstream a standard municipal WWTP, are evaluated and the impact of different technical choices is shortly discussed (disinfection options, chemicals, etc.). Some considerations about the link existing in Italy between the cost of water for irrigation and the agricultural economy are made.

Feasibility study to upgrade a textile wastewater treatment plant by a hollow fibre membrane bioreactor for effluent reuse.

A pilot plant membrane bioreactor has been tested in parallel with a full-scale activated sludge wastewater treatment plant fed on the wastewater from a textile factory. The possibility to upgrade the final effluent for internal reuse was investigated. The pilot and full-scale plants are located in a textile factory (Boselli & C., Olgiate Comasco, North Italy) which manufactures and finishes polyester fabric. The activated sludge wastewater treatment plant (WWTP) is an extended aeration system. The MBR pilot plant is a ZW-10 bench hollow fibre module (membrane surface area: 0.93 m2) submerged in a 200 L tank. Performance and operation of the membrane bioreactor (MBR) were evaluated in terms of permeate characteristics and variability (COD, colour, total N and P, microbiological counts), of membrane specific flux (l m(-2) h(-1) bar(-1)) and other operational parameters (sludge growth and yield).

Enzymatic and metabolic activities of four anaerobic sludges and their impact on methane production from ensiled sorghum forage.

Biochemical methane potential (BMP) tests were run on ensiled sorghum forage using four inocula (urban, agricultural, mixture of agricultural and urban, granular) and differences on their metabolic and enzymatic activities were also discussed. Results indicate that no significant differences were observed in terms of BMP values (258±14NmLCH4g(-1)VS) with a slightly higher value when agricultural sludge was used as inoculum. Significant differences can be observed among different inocula, in terms of methane production rate. In particular the fastest biomethanization occurred when using the urban sludge (hydrolytic kinetic constant kh=0.146d(-1)) while the slowest one was obtained from the agricultural sludge (kh=0.049d(-1)). Interestingly, positive correlations between the overall enzymatic activities and methane production rates were observed for all sludges, showing that a high enzymatic activity may favour the hydrolysis of complex substrate and accelerate the methanization process of sorghum.

Benefit of sodium hydroxide pretreatment of ensiled sorghum forage on the anaerobic reactor stability and methane production.

The assessment of the pretreatment effect on the anaerobic digestion process is generally based on the results of batch tests, which may fail in truly predicting full-scale anaerobic reactors performance. Therefore, in this study, the effect of alkaline pretreatment on the anaerobic digestion of ensiled sorghum forage was evaluated by comparing the results of two semi-continuous CSTR (Continuously Stirred Tank Reactor) anaerobic reactors. Results showed that an alkaline pretreatment step, prior to the anaerobic digestion of ensiled sorghum forage, can have a beneficial effect both in enhancing methane production (an increase of 25% on methane production was observed, if compared to that of untreated sorghum) and in giving more stability to the anaerobic digestion process.

Assessment of rheological methods for a correlation to sludge filterability.

To achieve good dewaterability, the conditioning process should create sludge flocs providing a porous cake during compression but also a high resistance to erosional stresses. This suggests that sludge's rheological characteristics, some of which (like the yield stress or the storage modulus) represent its network strength, should be related to the material's dewatering properties. Previous efforts to verify such a correlation--which would allow prediction of full scale dewaterability--have not provided strong correlations. In this work, commonly accepted rheological and dewatering tests were applied to conditioned sludges. Correlations were seen between different rheological parameters, particularly yield stress and specific energy, but not between rheological and filterability properties. Even using high pressure filtration tests, which should reflect the sludge's responses to normal and shear stresses, results were related to the measured rheological properties only in an indirect manner: a threshold shear strength is required beyond which further strength confers no improvement in filterability. Thus, common rheological tests are unlikely to provide useful information regarding full scale filterability.