Best practices of composting systems

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The HotRot system

The HotRot system is a horizontal in-vessel system used for composting a wide variety of organic wastes, including sewage sludge, grit and other screening materials from wastewater treatment plants, garden and kitchen waste and other municipal solid waste. HotRot is an enclosed U-shaped vessel with a central axial shaft. Capacity of the HotRot system is dictated by the residence time of the material to be composted in the HotRot unit. With HotRot’s ability to manipulate residence time via shaft rotation and increasing input quantities, plant capacity can be manipulated to compensate for short-term or systematic fluctuations, as for instance caused by 5 or 6-day working weeks or the receipt of specific wastes at specific times. The HotRot system utilizes a modular horizontal composting chamber; each chamber is individually controlled and monitored to ensure a consistent and predictable compost product. The material to be composted enters in one end of the HotRot unit where a central shaft mixes the waste and distributes moisture evenly. The central, tine-bearing, shaft is turned intermittently under programmable logic control (PLC) to provide both aeration and assist waste transport. Rotating the shaft both forward and reverse allows for independent process control of residence time and aeration The HotRot Composting System is fully modular, allowing plant capacity to be easily expanded as community organic waste separation and collection programs increase in efficiency. Modularity can also allow an organic waste treatment facility to be configured to produce niche products from specific feedstocks. Although the residence time in the system is variable depending on the waste feed rate and on the shaft rotation speed, it is usually adjusted to 15-18 days. Nevertheless, lower residence time can be implemented (Loizidou et al., 2006).

Examples using of HotRot system

  • Country: UK. Facility: Western Isles. The Western Isles composting plant is located in Scotland and is operated by the Highlands and Islands Council. It employs the HotRot in-vessel technology to treat organic waste and sewage sludge from anaerobic digestion. The plant is a small to medium scale plant, having an annual capacity of 4,000 tonnes.
  • Country: New Zealand. Facility: Bromley Plant. The Bromley plant at New Zealand employs the horizontal in-vessel system of HotRot for the treatment of sewage grit and screenings. The plant is under the operation of Christchurch City Council (CCC) and is operating since April 2003. It has installed the largest HotRot module (model 3518), which has a capacity of 10-12.5 tonnes per day. The annual capacity of Bromley plant is 4,350 tonnes.
  • Country: New Zealand. Facility: Army Bay, Aukland. The Army Bay composting plant is located in Aukland of New Zealand and is currently operated by the Rodney District Council. It is a small size plant that utilizes the horizontal in-vessel technology of HotRot. The system has a capacity of 550 tonnes per year and treats sewage grit and screenings generated from the nearby wastewater treatment plant.
  • Country: New Zealand. Facility: Palmerston North. The Palmestron North composting plant is a small-sized plant of New Zealand and is operated by the Palmerston North City Council. It treats grit and screenings from nearby wastewater treatment plants and has an annual capacity of 550 tonnes.

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Pilot implementation of our compost unit in Florina

In the current project a compost unit has been installed in Western Macedonia, in the regional unit of Florina in order to satisfy the needs of the local stakeholders as they have been recorded through the questionnaire ( in the symbiosis platform ( Below there is a description of how it works and which are the categories of biowaste.

The procedure

The biowaste are composted in 30-40 days, starting with shredding. After that, biowaste are driven to the first chamber, this of pre-processing/first maturation where for fifteen days and at predetermined intervals, it is stirred by a mechanical mixer (with wings) that the compost unit has, in order to provide it with the required oxygen. At the same time, on weekly basis, solid new composting materials to the compost unit are added. In the above first step, the compost unit transfers the new incoming waste to the first chamber. In the second step, which begins on 16th day, the biowaste of the first chamber is promoted to the second chamber where, for another 15 to 20 days at determined times, it is stirred by a mechanical mixer (with wings). The mature compost is then collected from the export valve.

Biowaste categories

The biowaste collected and used in the composting process belong to the basic categories of food, green and dry materials.

  • Fresh and boiled fruits and vegetables, coffee grounds, cooked fruit and fish skins but not fats and oils are used in the food category
  • Fresh herbs, flowers and leaves of the plant are used in the category of green materials
  • In the category of dry materials are used dried leaves and greens, coffee filters and ash

Figure  1 The compost unit in Florina


Loizidou, M., Malamis, S., Hapeshis, K., Kapetanios, E., Tsolomiti, G., & Malamis, D. 2006. Review of best practices and success stories of composting of sludge and other BOW in the EU and internationally. Deliverable 4 in the project: Design and Application of an Innovative Composting Unit for the Effective Treatment of Slunge and other Biodegradable Organic Waste in Morocco (MOROCOMP). Program: LIFE TCY05/MA000141.

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