There has been a lot of research into this topic but generally the temperature would need to be above 40C to kill pathogens and seeds - about 55C is recommended to kill weed seeds effectively (although this is normally only achieved at the very centre of the heap). If the temperature is much higher than this, the beneficial micro-organisms may be killed, stopping the composting process. Ideally you need to monitor the temperature at the centre of the heap with a thermometer and turn the compost regularly to encourage micro-organism activity. The type of organic matter that is added to a compost heap will also have a massive effect on the temperature of the heap.
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