Aerated Static Pile Composting
ASP composting is a method that uses a variety of systems to break down organic materials without the physical manipulation required in primary composting. These systems place the organic material into an air-filled chamber and provide controlled aeration. The following article describes the components of an ASP system and what they do.
ASP Systems aerated static pile (ASP) composting systems are designed for the efficient distribution of air within a compost pile. These systems utilize temperature feedback to balance air levels within the pile. Proper aeration reduces the amount of biological heat produced and increases the amount of oxygen present in the pile. However, too much aeration can slow down the composting process. The ASP Systems come with two blower control systems, the CompDACS controller for PC interfaces, and WebMACS for web-based operation.
In a study that examined the difference between static and vermicomposting systems, researchers created 12 piles, six of which were triangular in shape and were filled with equal amounts of organic wastes and cow dung. Three replications of the experiment were performed for each method. The static pile method required a 10-m-long compost pad and a slope to drain leachate.
The ASP Systems aerated static pile systems use a two-layer structure to control the moisture content of the pile. The plenum layer is made of wood chips or other compostable material. This layer diffuses the air and prevents aerated static pile composting it from going straight up or diverging to the sides of the pile. The active layer, meanwhile, is comprised of compost material that must maintain a 25:1 carbon-to-nitrogen ratio and reach a temperature of 131 degrees Fahrenheit for three days. In addition to providing a moisture-solubility control, the bio layer also discourages insects, reduces odors, and reduces the amount of heat released into the atmosphere.
Aerated static pile composting is a low-cost method of composting large volumes of organic waste. It is an excellent solution for facilities handling large volumes of food scraps, livestock manure, or yard debris. ASP systems are easy to use and provide a hands-off option for the composting process.
Air plenum layer
In aerated static pile composting, air flow is distributed across the pile through the use of a layer of coarse wood chips or screen overs. This layer also acts as a drain, absorbing excess moisture. The pile must be at least eight to ten feet high, but it can be higher.
The air plenum layer allows the pile to remain aerobic for longer, and it minimizes odor and anaerobic conditions. Another benefit of this technique is the reduced amount of manual work needed – it doesn’t require turning the pile, and it can even be adjusted according to seasonal climate changes.
The air plenum layer is a critical component of aerated static pile composting. This system consists of a grid of aeration piping connected to a blower. The air flows through the pile evenly. The active composting period lasts from three to five weeks. An air plenum layer is required to keep the pile at the right temperature for the microbes and organisms to thrive.
An aeration grid, usually made of 100-150mm perforated plastic pipe, is laid inside the 0.3-m (1-ft) thick plenum of wood chips. This layer helps to evenly distribute the air during the composting process, and absorb moisture that might condense. The resulting compost pile is then built on top of the plenum. Usually, the final compost is two to three metres tall and 150-200mm (6-8) thick.
The initial compost pile height should be at least 150 cm. It may need to be taller depending on the weather conditions and the equipment used to build it. This extra height helps retain heat during winter. Adding about 15 cm of finished compost to the top of the pile will prevent it from drying out, prevent flies from breeding and insulate the pile. This layer will also filter ammonia and potential odours.
Fan speed control strategy
Aerated static pile composting (ASP) uses static trapezoidal piles of heterogeneous feedstocks to compost organic wastes. The piles are aerated passively or forcedly with air. The composting process requires a bulking agent that is generally wood chips or other material. The bulking agent must be replaced in approximately three-thirds of the pile’s volume.
One way to regulate the airflow rate is to measure the amount of oxygen uptake in the pile. Increasing the oxygen uptake rate by a few percent can reduce the amount of energy required by up to 30 percent. Different wastes require different airflow rates; cow manure requires about 1000 L of air per kilogram of TS/d, while anaerobic digestate requires about 1600-1850 L per kilogram. The airflow rate is an important factor in composting as it is a significant energy input.
Dedicated fans are generally installed on a timer. Some systems also use a aerated static pile composting computerized control system to monitor fan speed. They must be carefully designed for peak air flow but typically operate at 15 to 30 percent capacity. While this can have a large impact on energy costs, it is important to note that the volume of air delivered by these systems is limited by the fan’s efficiency and system loss.
For community composters, an O2 Compost blower may suffice. For advanced operations, larger blowers are required.
Moisture levels in aerated pile composting vary by pile type and composition. When the mix of dry and wet ingredients is optimal, the pile has a higher water content and gains its water-holding properties faster than a pile with a lower moisture level. Moreover, it retains moisture longer, with lower temperatures and reduced evaporation rates.
While there are several reasons for this phenomenon, one is the movement of air through the pile. This movement is dependent on the size of the pile and the amount of “pore space” inside it. The amount of pore space in a pile is determined by the particle size, shape, and bulk density. The amount of moisture in a pile is also dependent on the amount of air present outside.
Compost microbes need an adequate supply of oxygen and moisture to survive and reproduce. In aerated static pile composting, the oxygen supply is obtained through convection and diffusion. Heat generated within an active pile enhances diffusion, which pulls in cooler air. However, it is important to maintain a high level of air circulation during the active phase, as too little oxygen can hinder the composting process.
While the main approach to moisture control in an aerated static pile composting system is to optimize the initial moisture content, there are several drawbacks to using this approach. One of them is that dry composted materials require a higher energy input, reduce the heat production per unit of composting material, and increase the occupying area of the pile. Another common method is the use of bulking agents, which have limited effects on initial moisture content adjustment but promote aeration.
In aerated static pile composting, the moisture levels decrease as the pile ages and decomposition progresses. The moisture content begins at about seventy-nine percent when the pits are filled, and decreases to less than half this level at the end of the process. During the final stages of the process, the ASP reaches the thermophilic phase with temperatures between 25 and 45 degC.
Solar power options
Solar power is an increasingly viable option for aerated static pile composting. In addition to being environmentally friendly, solar power is also affordable. The ASP Pilot Project can be easily installed and converted to solar power in phases, depending on the site’s needs. Solar power is an especially attractive option for sites that are not connected to the electrical grid.
Solar energy capture is essential for aerated static pile composting. The energy gained is directly related to the temperature differential between the compost vapor and the heat sink. During a four-hour trial, the difference between the compost vapor stream and the water in the heat sink tank increased.
In West Haven, Connecticut, Atlas Organics installed a modular EASP in a food waste diversion and compost pilot project that was part of the Natural Resources Conservation Service’s Urban Farming program. The system is capable of handling three aerated static pile zones, each zone containing 800 cubic yards of mixed material. In total, this system is expected to produce 3,000 cubic yards of compost annually. It also handles inbound feedstock including yard waste and food. The company also offers a modular EASP unit that provides remote control aeration and constant temperature monitoring.