DESC:Pre-digester unit for effective treatment of biodegradable fraction of solid waste

Field of Invention
The invention relates to dual phase anaerobic digesters. More specifically, the invention relates to first phase of the anaerobic digester where hydrolysis and acidogenesis takes place.

Background
Existing single stage biogas plants are usually unstable and unreliable. This is due to inherent issues with feedstock availability and quality. The day-to-day addition of feedstock is critical to the stability and reliability of the digester. There are complex logistic challenges in transport of waste, in the cases where the source of the waste is not the site for processing, as seen in decentralized and semi-decentralized biogas production facilities. This is primarily seen in and across urban cities where organic waste (organic fraction of municipal solid waste, OFMSW, fully source segregated and partially source segregated alike) is abundant, a good feedstock for producing biogas through anaerobic digestion, but with a challenge of consistency in availability and quality.

Anaerobic digestion (Biomethantion / Biogas production) is a series of biological processes in which microorganisms break down biodegradable material in the absence of oxygen. The four stages, hydrolysis, acidogenesis, acetogenesis and methanogenesis are distinct and have unique microorganisms which carry out the process.

The stability of a single phase digester of anaerobic digestion is prone to many problems, (especially for rapidly acidifying feedstock like food waste primary constituent of OFMSW) owing to the fact that all the biological processes are subject to the same conditions (temperature, pH, acidity, alkalinity, ambient gas levels, etc.),. There are various advantages of separating the four biological processes into two (or more than two) separate stages. The advantages focal around this application are, higher stability (as reflected by a lower acidity to alkalinity ratio in the final reactors) and the freedom to alter the physical locations of either or both digester(s) to optimize and subsequently minimize logistics costs and maximize waste processing capacity.

The majority of the organic fraction of municipal solid waste in developing countries is not source segregated, despite public policies, and is sent directly to landfills. To build a biogas plant within the premises of an urban setting handling the large amount of waste generated is infeasible due to space and safety constraints. By creating a separate pre-digestion reactor that contains the hydrolytic and acidogenic processes, volatile fatty acids can be synthesized and sent off to a remote location where the acetogenic and methanogenic processes occur. Thus we create two sets of users - users who want to process the waste and users who want to use clean and green fuel.

The majority of biogas plants in the past have been in a single phase digester, limiting the control on individual biological processes. In areas where a two stage system is established, the maintenance is focused specifically on the second phase i.e. acetogenic-methanogenic in-situ reactions. The process of hydrolysis and acidogenesis are generally robust and thus were not a priority to optimize in comparison to the acetogenic-methanogenic processes mentioned. Moreover, the existing multi stage systems are generally constructed in close proximity, this requires the daily transport of raw material to the site. A scenario can be expected such that at various sources, there can be a time interval between the waste generation and treatment/disposal, which would require provision of a buffer storage area where the waste will be subject to uncontrolled degradation and leachate generation. Also during transportation, to the processing/disposal site, uncontrolled degradation of organic material can be expected along with leachate generation etc. This is detrimental to environment and can alongside help the growth of organisms and attract pests. The unwanted emission can be controlled in a systematic fashion.

Object of the Invention
The object of the invention is to provide a pre-digester for effective treatment and processing of biodegradable fraction of solid waste for hydrolysis and acidogenesis which can be optimized.

Description of the Invention
The focus of the invention is on optimizing both the inherent reactions encompassing hydrolysis and acidogenesis as well as incorporating catalytic additives as addressed in the invention.

In urban settings, the construction of a large energy recovery from waste processing facility is infeasible primarily due to space restrictions and in some cases, health regulations. Splitting the process into two phases by developing the comparatively smaller pre-digester units all across the city that cater to a single (or few) digestion reactors in off-site locations, solves the space issue as well as the health issues.

The locations of the pre-digester units will be determined based upon surveys on where the highest organic fraction of municipal solid waste (OFMSW) is generated. In selecting a precise location, the logistical cost of transporting the OFMSW is reduced, and more importantly a significant portion of energy is recovered.

Another application that has been identified is the possibility of requirement of appropriate treatment of waste in the case a secondary digester (a digester for acedogenic and methanogenic processes) is unavailable. These sites can then house an aerobic post treatment unit, which will help with the reduction of parameters such as BOD and COD to allow for safe disposal in sewage systems / waste water treatment plants.

The apparatus is designed to enhance while closely monitoring the breakdown of complex organic matter in to Long Chain Fatty Acids and Volatile Fatty Acids by optimizing and controlling the pH, temperature, retention time and chemical/ enzyme addition, specific to the complex organic raw material.

For parameterization of the pre-digester unit, the pH, the temperature, oxidation-reduction potential (ORP), volatile fatty acid (VFA) content, as well as the propionic-to-acetic acid ratio are optimized by varying the hydraulic retention time (HRT) and organic loading rate (OLR).

Additionally, in one embodiment, the addition of various buffers and enzymes to enhance the microbial action on the feedstock will be tested as well as determination of the toxicity levels of various volatile fatty acids will be done.

The digestion in the tank can be assisted by manually/automatically adding cultures of microorganisms to enhance the biological processes. The addition of cultures may or may not be pre-scheduled and will be added upon investigation of the digestion efficiency.

The physico-chemical parameters of waste like total solid, volatile solid, total carbon, total nitrogen content, etc. are crucial to define design considerations for the pre-digester unit. In an embodiment of the invention, a pre-digester unit design shall be provided based on feedstock physico-chemical parameters of the input raw material. The desired output is to be identified to help define operational and designing parameters like HRT and OLR. Also, ORP, pH, temperature and VFA profile will help to operate pre-digester more efficiently and the same will be optimized and monitored.

The pH, ORP, VFA profile and temperature will be monitored using sensors built into the pre-digester unit and synced with a GPRS system to update the readings at predetermined time intervals – ensuring the optimum level of these three parameters. Depending on the requirement of waste, enzyme application for faster catalytic activity will be checked by conducting experiments determining the volatile fatty acid production on a daily basis in a continuous feeding assay. Determining the optimum enzyme addition will increase the total VFA content as well as the acetic acid-to-propionic acid ratio. The optimum hydraulic retention time will be determined by carrying out a similar continuous feeding assay with varying HRTs.

The end result of energy recovery is possible through a single phase digester or even a biphasic digester with both phases of the energy recovery process in the same location. However, the stability of the single phase digester does not match that of a biphasic digester. Secondly, building a two phase digester side-by-side solves the stability issue, however the possible space constraint then hinders the fabrication of a system large enough to process the entire waste generated by a specific area.

Although the innovation aims to solve the problem of stability and logistics of waste management in urban areas, it finds applications in rural and agricultural domains as well. The setup of pre-digester units at Cattle farms can reduce the GHG Emissions from storage and drying of cow manure. For industries such as breweries, food processing units and large industrial kitchens, waste management can be simplified and be monetized as well.

,CLAIMS:We claim:
1. An apparatus for processing organic biodegradable material into short chain fatty acids, comprises of:
a. A mechanical shredder/pulverizer,
b.A mixing tank
c. A suitable pump for transfer of material and mixing
d. A tank(s) adapted to process organic biodegradable material
e. An electronic system
f. A set of various sensors
g. A temperature management system
h. A scrubbing mechanism
i. An enzyme and/or chemical dosing system
j. An additional buffer tank

2. The apparatus according to claim 1, comprising of atleast one mechanical shredder/pulverizer which performs mechanical pretreatment of organic biodegradable material to reduce it into smaller particles so as to increase the surface area for accelerating and/or increasing digestion.

3. The apparatus according to claim 1, comprises of a tank (closed or open) used for mixing and temporary holding of the shredded/pulverized and/or processed material having multiple customizations for various applications such as removal of unwanted particles by settling/ floating particles, heating with/without temperature control.

4. The apparatus according to claim 1, comprises of atleast one pump suitable for transfer of shredded/pulverized and/or processed material, wherein thepossible types of pumps are positive displacement pumps and submersible cutter pumps.

5. The apparatus according to claim 1, comprises of at least one tank, in series or in parallel, made of any suitable material, with customizations in size, shape, orientation, heating arrangements, mixing equipments, sludge removal mechanisms, number and location of inlets and outlets, direct and indirect heating arrangements, acidic gas scrubbing mechanism to allow for appropriate processing of the raw material, or a combination of raw materials, selected based on their availability and physicochemical parameters, through biochemical processes such as hydrolysis and acidogenesis.

6. The apparatus according to claim 1, comprises of an electronic system that is capable of monitoring the defined parameters such as pH, temperature, Volatile Fatty Acid profile, Oxidation Reduction potential, liquid level, through a set of appropriate sensors placed strategically inside the tank described in claim 5 and has data transmission capabilities to send and receive data and instructions.

7. The apparatus according to claim 1, can have a manual/automatically controlled enzyme and/or chemical dosing system which comprises of a mechanism to measure the transfer of material into the tank, a small vessel to dissolve the enzyme and/or chemical if it is a soluble solid, a pump for addition of the material, with a possibility of manual addition of the material directly into the mixing tank, to finally achieve the desired pH and digestion parameters.

8. The apparatus according to claim 1, may receive raw material from multiple sources having multiple feedstocks and the treated raw material (also known as hydrolysed slurry) can be transferred to a single or multiple gas generation sites (having one or more digesters) which do not necessarily have to be in the same location, optimized as per the requirement.

9. The apparatus according to claim 1, may have an additional buffer tank which will be used to bring the Biological Oxygen Demand (BOD) and Chemical Oxygen Demand (COD) to lower levels using appropriate systems such as aeration systems and use of oxidative chemicals.