In-vessel Composting at NRS

Process Flow Chart

NRS Composting Flow Chart

Vessel Description

The composting of organic wastes at NRS is centred around the in-vessel system that uses five vessels. Each vessel is constructed of concrete walls and a base; with the base housing a plenum chamber that enables the supply of air up through a large number of distribution points in the floor of the vessel. Approximate external dimensions of the units are 25m long x 4.5m wide x 3m high.

A removable steel door is fitted and sealed to the front of the vessel. A flexible membrane is built-in to the top of the vessel to seal the walls and door and complete the total enclosure of the vessel.

Air is supplied to the composting mix in the vessel via a purpose built stainless steel fan and stainless steel ductwork system.

In-vessel Composting Process

The compost pre-mix of organic wastes that are of suitable moisture content and porosity is firstly loaded into the concrete vessel prior to the lid and door being put in place; this then forms a sealed chamber. Air is then introduced into the vessel via a myriad of distribution points within the concrete floor. After passing upwards through the mix, the air is collected in the headspace of the vessel via a fan and then blown back into the distribution chamber and up through the compost.

At this point, the system is operating under full recirculation conditions with no discharge to atmosphere. The continual recirculation of air through the compost bed serves to equalise, temperature, oxygen and moisture content within the mix; and the constant up-flow of air also serves to maintain the bed in a porous state and prevent excessive compaction.

The reservoir of oxygen within the system at this stage is sufficient to supply the demands of the developing microbes. This enables readily degradable organic compounds to be largely decomposed under totally enclosed, aerobic conditions. It is these types of compounds that would be most likely to contribute to odour generation.

As microbial activity accelerates, temperatures rise and oxygen levels drop; therefore the temperature of the entire bed is continually monitored in the air recirculation stream. A temperature in the 55-60°C range is maintained to provide pasteurisation of the mix and to favour the proliferation of a full range of composting microbes.

The temperature control system takes readings from the sensors in the recirculation line and feeds these to a PLC system. If the temperature exceeds the selected set point, the air damper system is automatically activated to bring fresh air into the system and to exhaust hot air, in order to cool the mix; hot air can be exhausted directly via a stack or to a biofilter. Once the temperature has returned to the desired point, the dampers automatically close and the process continues in the recirculation mode.

Similarly, if the oxygen level drops below a predetermined set point the air damper system is automatically activated to bring fresh air into the system for re-oxygenation.

Schematic

Process Control System

The entire NRS in-vessel composting cycle is under the management of a Citect System Control and Data Acquisition (SCADA) package that has been specifically programmed for the process in question.

Vessel Capacity

Each in-vessel unit has the capacity to accept approximately 300m3 of initial mix. Annual throughput for each vessel lies in the range of 4000t-6000t depending on the types of waste and the time that the mix is retained in the vessel.