9. DIGESTATE MANAGEMENT
As illustrated in the section about Wet Versus Dry Digestion, there are essentially five (5) types of digestate:
Clean liquid digestate
Clean solid digestate
Contaminated liquid digestate
Contaminated solid digestate
Contaminated semi-solid digestate
Only the clean liquid or solid digestates can be directly applied to land without further treatment. These digestates often originate from manure, food waste or pre-treated SSO digesters where there are virtually no contaminants in the feedstock to be digested.
Most of the time, liquid digestate is separated into solid and liquid fractions by using liquid/solid separation technologies.
As mentioned earlier, clean solid digestate can be applied to land directly.
Contaminated solids coming from a dry digester or a liquid/solid separation equipment will need to be composted to achieve proper dryness for the sieving of contaminants prior to land application.
Also mentioned earlier, clean liquid digestate can be applied to land directly.
Contaminated liquids coming from a wet digester or a liquid/solid separation equipment will require proper wastewater treatment such as sedimentation of suspended solids, abatement of COD, BOD and ammonia compounds.
10. BIOGAS PLANT COMPONENTS
Several aspects need to be studied in the choice of a site:
Dominant wind/Air dispersion
Proximity to energy grids
Proximity of neighbors
The biogas plant will be equipped with roads, scale, drainage, landscaping, etc.
Biogas plants will have one or more building(s) to contain the process and all the human resources operating and maintaining it. These building may require special architectural specifications for aesthetic, comfort and efficiency.
This is the area of the biogas plant where the feedstock is received. It may be designed to receive several trucks of various sizes. Typically, the reception of material will be indoor, and this is where most of the odor challenges arise. Opening and closing of doors for trucks is typically the main source of odors for a biogas plant.
Depending on the technology used, this is where the received material is prepared for feeding into digesters. It may be decontaminated by using technologies such as:
Central equipment of a biogas plant, the digester is where feedstock is biodegraded by anaerobic bacterias to generate the biogas and digestate.
Separation : The liquid digestate may be squeezed to separate the liquid fraction from the solids.
Composting : A biogas plant may be equipped with a composting plant to stabilize or allow the drying and decontamination of the solid fraction of its digestate.
Wastewater treatment: The liquid fraction of the digestate often needs to be treated prior to disposal into nature or into sewage.
Odors generated inside the building need to be controlled (with proper ventilation) and treated prior to rejection into the atmosphere.
The following list includes all the equipment necessary to handle the biogas:
Equipment necessary to clean the biogas to the proper specifications for the intended application.
Equipment that will allow utilization of the biogas or biomethane (RNG) as follows:
11. BIOGAS PROJECT ECONOMICS
Biogas plant economics are complex and vary with local market conditions.
Biogas plants can generate several revenues such as:
Treatment fees: Money you receive (or save) for accepting and treating the feedstock. In North America, this represents the majority of the income of the project (60-80%).
Energy sales: Money you receive for selling the biogas energy (20-40% of income). Only in markets with generous feed-in-tariffs will the energy sales constitute the majority of the project income.
Digestate/compost sales: Money you receive (or save) for selling your digestate or compost. Typically, you have to pay to dispose of the digestate or compost.
Carbon credits: Biogas plants do generate carbon credits that can be sold. However, the volume is small, and the validation and certification fees often take the lion’s share of this income.
Biogas plants are financed using equity, debt, subsidies and tax credits.
Municipalities will finance their project with subsidies and debt. Private projects will require significant equity (25%) and energy contracts from solid clients to secure their debt.
Operational expenses (OPEX) are typically composed of:
Debt service charges
Disposal charges (contaminants, digestate)
Capital expenses (CAPEX) vary greatly between projects.
Municipal projects are the most complex and expensive. Typically, in North America, they cost anywhere between $800-$1500/tonne of annual treatment capacity.
Agricultural projects are the simplest and least expensive. Typically, in North America, they cost anywhere between $4500-8000/kW electrical installed.
12. BIOGAS PROJECT DEVELOPMENT
There exist several critical steps in the realization of a successful biogas project, though project developers tend to focus their effort on determining the best anaerobic digestion technology for their project instead of getting a firm grip on their project fundamentals before anything else.
Biogas plants are large expensive finicky biological systems that require careful planning. In fact, most biogas plant failures are due to poor planning and/or not paying close enough attention to project fundamentals such as feedstock, energy utilization, digestate management, and financing.
STUDIES & PRELIMINARY ENGINEERING
A lot of work must be put into establishing the project fundamentals (studies).
Establishing expected feedstock collection methods (trucks, bins, routes, etc.), quantity, quality and overall logistics (collection contracts, transfer stations, hours of reception, etc.) often require significant studies and planning. One must not underestimate the effort necessary to understand how much, when, and in what state the organic waste will get to the biogas plant.
Finding a proper site for a biogas plant also requires significant effort. The site needs to meet proper zoning, and environmental regulations (proximity to houses, rivers, wells, etc.). The site must also be easily accessible by road for the feedstock to come in, and the digestate to come out without causing too much traffic nuisance to the neighborhood. Finally, the site must be close an energy grid (gas or electrical) in order for the biogas energy to be exported efficiently.
Digestate management must be studied carefully since the disposal of digestate is often the largest operational cost of a biogas plant. All possible avenues of disposal, transformation or treatment must be taken into consideration to ensure that the final strategy for digestate management is the most efficient. Otherwise, the biogas plant economics will be less than optimal.
Once the fundamentals are established, a concept will be drawn and priced to get a project budget.
Beyond this initial engineering concept, further studies and analysis are often required as follows:
Site contamination (soil, buildings, etc.)
Applicable codes, rules and regulations
Proper preliminary engineering is essential to develop a viable business case that will justify the significant financing required to realize the biogas project.
Detailed engineering of a biogas project is composed of several disciplines working in close collaboration such as:
To ensure that all engineering disciplines are working together closely to efficiently deliver an optimal design.
To determine the processes required for feedstock conditioning, anaerobic digestion, gas treatment, digestate treatment, odor management, etc.
To deal with all aspect of material handling: solid waste reception & conveying, liquid pumping, gas compression, etc.
To deal with all aspects of power supply and automation (sensors, PLCs and actuators).
To handle excavation, filling, and utility services (drainage, sewer, water, etc.).
To ensure foundations are safe and sound to support the structural load of the building bearing the process equipment.
Building Mechanical Engineering
To handle all aspects of ventilation, fire protection, lighting, non-process electricity and plumbing.
To handle all technical and visual aspects of the site and buildings.
Detailed engineering is performed to generate drawings and establish specifications for all the components of the biogas plant. The design must obviously comply with local codes and regulations.
PERMITTING & ENERGY CONTRACTING
Once the drawings and specifications are completed, the project must obtain all necessary permits for construction. Depending on the jurisdiction, there are usually several different authorizations required from local municipalities and environmental agencies.
In parallel, an energy contract should be negotiated with the local energy provider. These contracts can be technically and legally complex and will require proper technical and legal support.
Do not underestimate the time required to perform permitting and/or negotiate an energy contract with energy providers.
Financing will only occur if the project is permitted and if there is a serious client for the biogas energy.
Equity and guarantees will be demanded by financiers. Due diligence will be performed on the design, the clients, the management, risk analysis, etc.
Only upon satisfying all these answers will the project funding be confirmed.
Purchasing the products and services to realize the design. Typically, the procurement of the plant will be broken down into several contracts, such as:
Quality control labs (materials)
Process equipment (digesters, gas upgrader, hydropulper, conveyors, etc.)
Public entities, such as a municipality, will often issue a request for proposals (RFP) for a design-build (DB) or a design-build-operate (DBO) so that all these procurement contracts are performed by the chosen contractor. Municipal procurement is often cumbersome and slow, thus one should expect significant potential delays in the realization of the biogas project.
Once all the permits are granted and the financing obtained, the procurement and construction can begin.
Proper construction management, supervised the general contractor, is essential to ensure that procurement and the execution of the various contracts are well timed to avoid construction conflicts and unnecessary delays.
Engineering supervision is necessary to ensure that the constructions are in accordance with the design.
Construction sites must be managed properly to ensure security and safety of the workforce. The site must be able to accommodate temporary services (electricity, sanitation, accommodations, etc.) as well as material and equipment reception (laydown) and storage.
Upon completion of the various phases of the biogas project, pre-operational verifications must be performed to ensure that all equipment was properly installed. At this point in time, a partial acceptance of the biogas plant can be granted so that the various contractors can get paid.
After verification, the anaerobic digestion process may be started. There must be a proper coordination with the feedstock collectors to ensure they can sufficiently and efficiently supply the process with the feedstock.
Upon achievement of the performance of the biogas plant, a final acceptance may be granted to pay the balance of suppliers and officially begin the operation of the biogas plant.
Day-to-day operation of the biogas plant includes the following tasks:
Reception of Material
Operators will coordinate the logistics of feedstock arrival, perform visual inspection of the received material, and log tonnages received.
Conditioning of Material
Operators will transfer the material from the reception to the conditioning equipment.
Anaerobic Digestion Process Control
Operators will operate and monitor the various aspects of the anaerobic digestion process, such as temperature, OLR, FOS/TAC, pH, etc.
Operation of Digestate Treatment System
Operators will attend to the dewatering, drying, and water treatment processes.
Operation of Wastewater Process
Operators will ensure that the water treatment process is functional.
Disposal of Contaminants
Operators will manage the logistics and disposal of all contaminants generated by the process.
Operation of Composting Process (if there is)
Operators will operate and monitor the various aspects of the composting process.
Disposal of Digestate/Compost
Operators will manage the disposal of the digestate/compost.
Biogas plants are equipped with multiple equipment that must be maintained to remain optimally functional throughout their entire planned life cycle.
Also, operators must establish and perform preventive maintenance on the equipment.
Unplanned maintenance and repairs are also to be expected and required daily.
Optimization of the biogas plant may be achieved by performing modifications to improve processes or performance.
More Generated Power from Waste
Advance technology + automation + experience + expertise = Innovation
We can do the following for you:
– Generate more excess power to sell.
– Ceate more business revenue.
– Solve your environmental wastewater problems.
– Manage organic waste stench and storage.
– Extend boiler fuel inventory.
– Start a zero-waste system without really trying.
– Offer leading integrated W2E technology.
– Raise efficiency of power generation system.
We are PlanET Biogas Global GmbH. We have designed, engineered, and constructed over 430 biogas and biomethane plants all over the world. We can help you quantify, qualify, and harness the potentials of ALL your organic waste.
Biogas and Biomethane. Turning waste into resources.
Jess C. Gregorio
PlanET Biogas Global GmbH
Biogas Reference Site: