What wastes can be processed in the VCU®?
The VCU® can treat virtually any organic waste. VCUs have been installed to process, food waste, fruit, vegetables, fish and poultry wastes, sewage grit and screenings, green waste, source segregated green and kitchen waste, and many others. There are certain parameters to regulate, such as moisture content which should be around 50-60%. Also, particle size, the nutrient composition and pH of the material will affect the speed of decomposition. For certain wet wastes such as food and sewage sludge it may be necessary to either dewater or add relatively dry matrix materials. What is the capacity of the VCU®? The VCU® is particularly well suited for organic waste streams typically in excess of 2.5 tonnes per day. The VCU’s modularity means that multiple units can be installed to form a flexible plant capable of processing up to 160 tonnes per day. The throughput depends on the type of feed and how stable the product needs to be. Food and animal wastes will normally compost faster than wastes containing a high proportion of woody green-waste. The VCU® system can be used to provide pre-composting prior to windrow maturation or as a complete stabilisation process.
Are VCUs housed in buildings or outside?
The VCUs themselves are completely weatherproof and do not require a building, however if required the chambers could be installed internally.
Are odours a problem?
The VCU® is a self-biofiltering system. Odorous gases that may be produced in parts of the pile are consumed by microbes before reaching the exhaust outlet. A low level of exhaust gas is produced and this is primarily made up of carbon dioxide, water vapor and nitrogen. Some of the water vapor will condense on the walls of the exhaust ducts and is collected in a PVC pipe which runs the length of the VCU® system. Due to the dynamics of the process the condensate is usually benign and suitable for sewer disposal or irrigation.
By not injecting air into the VCU® system are anaerobic conditions, and therefore unpleasant odours, created within the pile?
Despite achieving high temperatures, the VCU® system consistently controls odour emissions, both within the exhaust gases and the harvested product. This signifies that the passive aeration method attained through the design of the VCU® system allows sufficient aeration through the pile, maintaining aerobic conditions and thus preventing malodours. Gaseous measurements taken 600mm below the top of the pile (hot zone) from a VCU25 have given consistent readings of 6 – 7% oxygen (verified by independent consultants).
Are contaminated wastes a problem?
The VCU® can easily cope with contaminants such as plastic bags, cans and glass. These materials can either be removed from the waste at the front-end, or from the processed compost by screening. . There are VCUs currently processing contaminated organics from a Materials Recovery Facility (MRF).
Is an inoculant added to the VCU® system to promote microbial activity?
No. The VCU® chamber is designed to optimise growth conditions for bacteria and fungi already present within the organic wastes and borne on the airflow through the mass. Design factors include insulation of the cell walls, design of the chamber to encourage convective airflow and preparation of the waste input blend to optimise the moisture content and carbon to nitrogen ratio. The addition of matrix materials also optimises airflow by increasing porosity of the pile. These conditions all assist in promoting both cellular and population growth of the microorganisms.
Conventional composting wisdom states that temperatures above 60°C inhibit microbial activity and therefore slow biological degradation, heat production and moisture removal. Do the high temperatures found in the VCU® chamber inhibit the composting process?
Microbiology is an emerging science with ongoing and significant increases in knowledge, particularly as new methods for culture and analysis are developed. Studies of microbial activity in hot composts, between 65 and 82° C, have found high numbers of bacteria present (107 to 1010 cells per gram [dw]) (Beffa et al., 1996).1 Studies have also suggested that these hyperthermophilic populations, often found in geothermal environments, play an important role in organic-matter degradation.
Insulative effects of an in-vessel system have also been said to create a biological constraint, with high temperatures that suppress biological activity persisting throughout the entire column. Does this occur in the VCU® chamber?
Again, ongoing research is in disagreement with the traditional belief that high temperatures suppress microbial activity. The VCU® system also offers a stratified temperature profile throughout the pile. Air is drawn up through the base of the VCU® allowing some cooling of the lower zone and resulting in a temperature profile that ranges from around 40 – 50 ° C at the base to 70° C or more in the upper zone. In this way the VCU® system allows the waste input to pass through a range of temperature zones, therefore undergoing treatment by an extremely diverse range of microbial populations and activity.
Are there regulatory requirements to meet to operate a VCU® system?
This will depend on the type of waste to be processed, the volume and ownership of the waste. For commercial composting operations a waste management license would be a minimum requirement, and compliance with ABP Regulations would be needed if processing Animal By Products. VCU Europa can assist in obtaining or supporting applications for permits.
Is it true that vertical in-vessel systems have a critical pile height, where backpressures hamper ventilation attempts?
How does this apply to the VCU®? Examples of vertical systems where ventilation attempts have failed are referring to processes that involve air injection. The VCU® system operates under convection airflow, where higher temperatures within the upper portion of the pile draw cool air up through the exposed base. Matrix materials are added to the waste input mix to maintain porosity within the VCU® chamber. Insufficient porosity would result in anaerobic conditions, signified by leachate and malodours. The performance of VCU® systems demonstrates that passive aeration/convective airflow is a successful method of aerating the 5m vertical pile.
What are the criteria for successful pathogen destruction and does the VCU® system meet these requirements?
Current European and UK composting directives are very much in support of high temperature composting as the most appropriate method to destroy pathogens. Current EU directives require composting under minimum time and temperature conditions of 70° C for 60 minutes. Attainment of these upper temperatures is an advantage of modern in-vessel composting systems such as the VCU®, overcoming problems associated with windrow/static pile systems, which experience cooling in outer layers. These directives are supported by scientific research, with a number of studies undertaken to assess the microbial activity present during high temperature composting.
What is the ideal particle size for the VCU® process?
In general the smaller the particle size the faster the composting process. As a guide, all particles including green waste should be less than 80mm in length. To meet EU ABPR, waste must measure either 12mm minus or 60mm minus (in one plane) depending on how the VCU® is to be operated. Often investment in the shredding of waste upfront of the VCU® will increase the economical viability of the process by reducing the size of the VCU® processing capacity.
How is waste fed into the VCU®?
Integrated feed systems specifically designed for each installation are supplied as part of the package. A feed system usually comprises of a mixer blender and an inclined feed conveyor which are controlled as part of the overall control system.
What does the term matrix refer to and why is it so important to the process?
Matrix is the material used to maintain porosity within the VCU® chamber, thereby creating air voids and pathways for the flow of convective air currents. Matrix materials may be part of the waste stream or an additional element added in the blender. An essential requirement of the matrix is that it contains large particles that will not be easily compacted beneath the weight of the pile. Materials typically used as matrix include shredded greenwaste, woodchip or a suitable synthetic material that meets with the dimension criteria.
What is the residence time in the VCU®?
The VCU® operates on flexible cycle times which typically vary between 4 and 14 days. Through all of these cycles an odour-stabilised and pasteurized product is produced. The cycle time will determine the maturity of the final product.
Does the compost need to be matured?
On a 7-14 day cycle the harvested end product can be immediately used as a surface mulch that can be spread onto the soil and turned/ploughed into unplanted ground. The mulch will improve the soil’s structure, increase the availability of nutrients, increase resistance to soil-borne plant diseases and improve air porosity, water holding capacity, nutrient storage and cation exchange capacity (CEC), and last but not least soil bio-activity. However, to be utilized as a compost/soil conditioner it requires a further period of maturation, usually from 4 to 6 weeks. On longer cycles the product is more mature and can either be used immediately or cured for a shorter period to yield a nutrient enriched compost.
Is it necessary to screen the compost?
This will depend on the size of the material entering the process. If it is small enough then screening may not be required. However, generally screening is required if larger particles are composted or the material is contaminated.
What are the operating costs of a VCU® site?
This is site specific and VCU Europa use a financial model that can be worked through with potential customers that takes into account the many variables on site.
What operational measures are taken to ensure the success of the VCU® process?
The key factor in the performance of the VCU® system is the preparation of the waste input materials. The wastes must be well mixed, with sufficient matrix added to maintain porosity within the pile. The various waste inputs are assessed and combined to achieve a moisture content of 55-60% and a carbon to nitrogen ratio of 30:1. VCU Europa Ltd recognizes the importance of the operator’s role in maintaining process performance and product quality. For this reason VCU Europa Ltd provide an in-depth training and operator certification programme and ongoing technical support. Operators are trained to monitor the composting process through the measurement of pile temperature, which is recorded at a range of heights, and various product characteristics.
What are the maintenance requirements of a VCU®?
As part of the operator training, VCU Europa provide a TPM (total productive maintenance) schedule. TPM is the next step beyond preventative and predictive maintenance programs. It utilizes the mechanical aptitude and experience of the operator to identify any changes in the operation of the plant before they cause a problem. VCU Europa Ltd offers preventative maintenance and support contracts as well as comprehensive back-up through the life of the system.
Are temperatures measured and recorded?
Each VCU® incorporates a number of temperature probes down the side of the chamber. These temperatures are continually monitored and logged by the control system. The built in data logger allows the operator to pull up a map of the temperatures measured on each probe, the level within a cell, the fan speed and the number of hours that the temperature has been above the required threshold within a 24 hour period. This log allows for an overview of the trends, particularly in temperatures, which can help with anticipating feedstock abnormalities before they become problematic. These records can be accessed on site and automatically sent to an off-site operator and/or regulatory authority.
1 Beffa, T., Blanc, M., Lyon, P., Vogt, G., Marchiani, M., Fischer, J. L. and Aragno, M. 1996. Isolation of Thermus Strains From Hot Composts (60 to 80° C), presented in Applied and Environmental Microbiology, May 1996, P. 1723-1727.