Residual municipal solid waste as co-substrate at wastewater treatment plants: An assessment of methane yield, dewatering potential and microbial diversity.

Separately collected organic fraction of municipal solid waste, also known as biowaste, is typically used to fill the available capacity of digesters at wastewater treatment plants. However, this approach might impair the use of the ensuing digestate for fertilizer production due to the presence of sewage sludge, a contaminated substrate. Worldwide, unsorted municipal solid household waste, i.e. residual waste, is still typically disposed of in landfills or incinerated, despite its high content of biodegradables and recyclables. Once efficiently separated from residual waste by mechanical processes, the biodegradables might be appropriate to substitute biowaste at wastewater treatment plants. Thus, the biowaste would be available for fertilizer production and contribute to a reduction in the demand on non-renewable fertilizers. This study aimed at determining the technical feasibility of co-digesting the mechanically separated organic fraction of residual waste with sewage sludge. Further, key parameters for the implementation of co-digestion at wastewater treatment plants were determined, namely, degradation of the solids and organics, specific methane production, flocculant demand, and dewatered sludge production. The microbial community and diversity in both mono- and co-digestion was also investigated. Semi-continuous laboratory scale experiments showed that the co-substrate derived from the residual waste provided a stable anaerobic co-digestion process, producing 206 to 245 L of methane per kg of volatiles solids added to the digester. The dewaterability of the digestate increased by 4.8 percentage points when the co-substrate was added; however, there was also an increase in the flocculant demand. The specific dewatered sludge production was 955 kg per ton of total solids of co-substrate added to the digester. Amplicon sequencing analysis provided a detailed insight into the microbial communities, which were primarily affected by the addition of co-substrate. The microbiota was fully functional and no inhibition or problems in the anaerobic digestion process were observed after co-substrate addition.

Mechanical separation of impurities in biowaste: Comparison of four different pretreatment systems.

Impurities in biowaste, such as plastics, glass, metals and inert material, negatively impact the operation of anaerobic digestion plants and compost quality, and have to be removed prior to the anaerobic digestion process. Different mechanical pretreatments are available for this purpose. However, data on the removal efficiencies of pretreatment systems for different types of biowaste and for different kinds of impurities are still scarce. This study aims to determine the efficiencies for impurity removal of four biowaste pretreatment plants (BTPs) at real scale - (1) drum-screen + shredder + piston press; (2) shredder + piston press + screw press; (3) separation-mill; and (4) pulper + drum-screen. BTP 1 treats mixed food and garden wastes, while BTP 2, 3 and 4 treat mostly food waste. The efficiency of the pretreatment systems was influenced by the type of pretreated biowaste. The recovery of organics by the press was more efficient when pretreating food waste (BTP 2, 93%) than for treating mixed food and garden wastes (BTP 1, 77%). BTP 3 presented the highest recovery of biogas (up to 98%), but also the highest transfer of inert particles to the substrate. On the contrary, BTP 4 was the most efficient for the removal of inert particles; however, this system also presented 18% loss of biogas potential.

Hydrocyclones for the separation of impurities in pretreated biowaste.

The aim of the mechanical pretreatment in case of anaerobic digestion of biowaste is to produce a substrate without impurities. To facilitate a failure free operation of the anaerobic digestion process even small impurities like stones or sand should be separated. As a result of an insufficient pretreatment or impurities separation, plant malfunctions, increased equipment wear or pipe clogging are reported. Apart from grit chambers or pulper systems, a hydrocyclone is a cost-efficient and space-saving option to remove impurities. The aim of this work was to investigate the efficiency of hydrocyclones for the separation of impurities. Two hydrocyclones at two different plants were investigated regarding their capability to separate the small inert impurities from pretreated source separated biowaste. In plant A, the hydrocyclone is part of the digester system. In plant B, the hydrocyclone is part of the biowaste pretreatment line (after milling and sieving the biowaste) before digestion. Separation rates of inert impurities such as stones, glass and sand were determined as well as the composition of the concentrated solids separated by the hydrocyclone. Due to the heterogeneity of the biowaste the impurity separation rates showed variations, therefore the following mean results were obtained in average: the investigated hydrocyclones of plant B, part of the biowaste treatment, separated more than 80% of the inert impurities in the waste stream before anaerobic digestion. These impurities had a size range of 0.5-4mm. The hydrocyclone integrated in the digester system of plant A showed separation rates up to 80% only in the size range of 2-4mm.

Impurities in pretreated biowaste for co-digestion: A determination approach.

Although the mechanical treatment of source separated organic waste typically includes processing steps to remove impurities like plastic bags, smaller particles like glass, stones or sand are often not sufficiently removed. These particles lead to plant malfunctions, increased equipment abrasion and accumulation in the digester. It is possible to remove these small impurities before or during the fermentation process but this requires additional equipment at the waste treatment facilities. For pretreated biowaste with fairly low concentrations of impurities and small particle sizes no appropriate method was found in literature to determine these particles. Therefore various approaches to develop an appropriate method were tested and finally one method was selected. Sample mass calculation showed that for the determination of impurities >2mm a sample mass of about 6kg is required to receive statistically sound result. Firstly an elutriation step is used to concentrate the impurities in a sinking fraction, still containing some organic material. The elutriated material is then dried. After drying the elutriated material, impurities can be fairly easily sorted manually. The elutriation process is applicable for the determination of impurities >1mm. Due to the difficult manual sorting of particles <2mm and the reduced sample mass required for the determination of particles <2mm, these particles are determined by a different procedure: A sample mass of about 1kg is dried and combusted in a muffle furnace. The remaining ashes are sieved from 2 to 0.06mm. Particles <0.06mm were not considered as impurities. The data regarding the impurities content and particle size distribution in food- and biowaste are required for assessing separation options as well as the behavior of stones or sand in the digester. This allows describing the quality of the pretreated biowaste. Furthermore the need to adopt or improve the existing pretreatment can be identified and the impact to the fermentation process (impurities accumulated in the digester, etc.) can be evaluated.

Waste Separation Press (WSP): a mechanical pretreatment option for organic waste from source separation.

An efficient biological treatment of source separated organic waste from household kitchens and gardens (biowaste) requires an adequate upfront mechanical preparation which possibly includes a hand sorting for the separation of contaminants. In this work untreated biowaste from households and gardens and the screen overflow >60mm of the same waste were mechanically treated by a Waste Separation Press (WSP). The WSP separates the waste into a wet fraction for biological treatment and a fraction of dry contaminants for incineration. The results show that it is possible to replace a hand sorting of contaminants, the milling and a screening of organic waste before the biological treatment by using the WSP. A special focus was put on the contaminants separation. The separation of plastic film from the untreated biowaste was 67% and the separation rate of glass was about 92%. About 90% of the organics were transferred to the fraction for further biological treatment. When treating the screen overflow >60mm with the WSP 86% of the plastic film and 88% of the glass were transferred to the contaminants fraction. 32% of the organic was transferred to the contaminants fraction and thereby lost for a further biological treatment. Additionally it was calculated that national standards for glass contaminants in compost can be met when using the WSP to mechanically treat the total biowaste. The loss of biogas by transferring biodegradable organics to the contaminants fraction was about 11% when preparing the untreated biowaste with the WSP.