flushing water with urine and feces. They require more involvement by the user than the "flush and forget" approach of flush toilets connected to sewage treatment plants. Urine-diverting dry toilets Composting toilets, although similar to and sharing many advantages and disadvantages with urine-diverting dry toilets (UDDT), are more complex and require more maintenance to keep a consistent and relatively high moisture content. Some composting toilets are designed with urine diversion. Types Commercial units and construct-it-yourself systems are available. Variations include number of composting vaults, removable vault, urine diversion and active mixing/aeration. Slow composting (or moldering) toilets Most composting toilets use slow composting which is also called "cold composting". The compost heap is built up step by step over time. The finished end product from "slow" composting toilets ("moldering toilets" or "moldering privies" in the US), is generally not free of pathogens. World Health Organization Guidelines from 2006 offer a framework for safe reuse of waste, using a multiple barrier approach. Slow composting toilets employ a passive approach. Common applications involve modest and often seasonal use, such as remote trail networks. They are typically designed such that the materials deposited can be isolated from the operational part. The toilet can also be closed to allow further mesophilic composting. Slow composting toilets rely on long retention times for pathogen reduction and for decomposition of waste or on the combination of time and/or the addition of red wriggler worms for vermi-composting. Worms can be introduced to accelerate composting. Some jurisdictions of the US consider these worms as invasive species. Active composters (self-contained) "Self-contained" composting toilets compost in a container within the toilet unit. They are slightly larger than a flush toilet, but use roughly the same floor space. Some units use fans for aeration, and optionally, heating elements to maintain optimum temperatures to hasten the composting process and to evaporate urine and other moisture. Operators of composting toilets commonly add a small amount of absorbent carbon material (such as untreated sawdust, coconut coir, or peat moss) after each use to create air pockets to encourage aerobic processing, to absorb liquid and to create an odor barrier. This additive is sometimes referred to as "bulking agent". Some owner-operators use microbial "starter" cultures to ensure composting bacteria are in the process, although this is not critical. Vermifilter toilet A vermifilter toilet is a composting toilet with flushing water where earthworms are used to promote decomposition