The carbon: nitrogen (C: N) ratio one is one of the key factors in aerobic composting along with the moisture content and the availability oxygen . More information on the Compost
C: N Ratio page.
In the case of food waste, which is high in nitrogen, there will be an additional need for a carbon source to provide the composting microorganisms with an energy source. In addition, food
waste has high moisture content and as it is broken down, water is released into the composting vessel. As a result, the structure of the composting material will not contain sufficient air spaces for aerobic composting as the food decomposes into a
wet, smelly sludge. This sludge further restricts, or prevents, airflow within the material leading to anaerobic fermentation and more unpleasant smells including hydrogen sulphide (rotting eggs). This attracts rats and flies making it particularly
important for the addition of bulking agent in school food composting to be carefully managed as if kitchen waste is added without bulking agent the compost will become a stinky mess within a couple of weeks.
the waste has become saturated, the smelly liquid will drain out of the bin onto the pad or into the soil.
To avoid this situation the bulking agent needs to add structure (or bulk) to the compost and create air spaces allowing
air to circulate within the composting material. The correct bulking agent will also absorb the liquid from the food to maintain the correct moisture content of the food waste during composting. To absorb the liquid effectively the bulking agent should be
Bulking agent should also be used to cover the composting food waste in static bins.
As the name implies the bulking agent also provides the “bulk density” when composting
food waste. The bulking agent should contain, and hold, plenty of free air spaces (FAS) and will have a low bulk density. Green Mountain Technologies  suggest
that the typical bulking agent should have a bulk density of less than 600 lbs per cubic yard and that wood shavings are a classic example at about 400 lbs per yard. Regular aerating the material is not a substitute for using a bulking agent containing free
air spaces as the turning process only introduces sufficient oxygen to support bacterial growth for a period of a few hours.
In addition to wood shavings a wide range of materials can be used as bulking agents during
home composting including wood chips, saw dust, wood pellets, dry deciduous leaves, shredded garden waste, shredded paper, shredded cardboard and animal bedding. In agriculture, other readily accessible materials such as hay, wheat straw
and corn stalks may be used. In some cases, animal dung may also be used.
The best bulking agent to use will depend on the particular composting container, method and the nature of the food waste being used. The quantity
required will also vary. Many recommend two parts bulking agent to one part food waste.
The following materials are listed in descending order based on my personal preference.
Wood chips can often be obtained for free from owners of land where trees have been felled, from landscape gardeners or be purchased from garden centres, It can be used fresh but composted wood
chip makes a better bulking agent. Their purpose is to promote natural airflow in the pile. Some wood chip may survive composting and if the compost is sieved it can be reused.
Wood pellets are effective but relatively expensive (about £2-£4 per week) for use as bulking agent when compared with free woodchip. However, they can be ordered over the net and the bags are delivered directly
to the door. The pellets are normally about 4cm long and 8mm in diameter.
Wood shavings should be used with care as they may form clumps when wet that can
restrict airflow within the composting material.
Sawdust consists of relatively small particles when compared with wood chip or shavings but due to its
size is very poor in providing air circulation when wet. It is a good source of carbon but will need regular turning to allow air circulation. It is possibly best used an absorbent in emergencies or in small amounts with another bulking agent. It also makes
a good initial absorbent when composting fruit pulp.
Hay and straw are often available after use as animal bedding. It is a good carbon source but is likely
to mat and restrict airflow when wet.
Regular and frequent turning is suggested to break up any matting material.
Shredded Paper and cardboard.
Shredded paper and cardboard may be added to conventional compost bins as a source of carbon and if crunched up to provide air spaces. However, they are not adequate as a bulking agent for home composting food.
Leaves and Garden Waste
Dry leaves and garden waste can be used and are best shredded but they have been used when cut into small pieces.
Food using “Garden” Tumbler
The production of leachate in tumbler systems, particularly when composting waste food, can complicate the composting if it seeps out of the tumbler onto the bin stand
or ground attracting flies and other peats,. The optimum C/N ratio for rotary drum composting ranges from of 1:20 to 1:25 compared with the usual recommended range for conventional compost bins of about 30/1 Sufficient bulking agent such as woodchip
or wood pellets being my must be added prevent leachate production with woodchip or pellets being my personal preference. Although where the fruit pulp is being composted, absorbing it with sawdust is advised as an initial stage.
Details of tumbler food composters are given below.
 (http://compostingtechnology.com/its-all-about-the-bulking-agent/ )