An environmental control system (ECS) having constituents in supply air that flows into an environment for enclosing occupants, wherein the supply air only includes conditioned air from outside air and recirculated air from the environment, comprising: an air conditioning pack that receives outside air and produces the conditioned air; a mix manifold located, in terms of airflow direction, upstream of the environment and downstream of the air conditioning pack, to mix the conditioned air from the air conditioning pack with the recirculated air from the environment and to provide the supply air; a fan located, in terms of air flow location, intermediate the environment and the mix manifold to move the recirculated air from the environment and to the mix manifold; and a regenerative treatment subsystem which includes treatment beds that adsorb and desorb constituents in at least the recirculated air before the recirculated air flows back into the environment; characterised in that the regenerative treatment subsystem is located, in terms of air flow direction, in one of a: first location directly downstream of the environment, whereby there is an absence of a system component between the treatment subsystem and the environment to affect air flow, except for a means to allow air flow between the treatment subsystem and the environment; and second location directly upstream of the environment, whereby there is an absence of a system component between the treatment subsystem and the environment to affect air flow, except for a means to allow air flow between the treatment subsystem and the environment. an air conditioning pack that receives outside air and produces the conditioned air; a mix manifold located, in terms of airflow direction, upstream of the environment and downstream of the air conditioning pack, to mix the conditioned air from the air conditioning pack with the recirculated air from the environment and to provide the supply air; a fan located, in terms of air flow location, intermediate the environment and the mix manifold to move the recirculated air from the environment and to the mix manifold; and a regenerative treatment subsystem which includes treatment beds that adsorb and desorb constituents in at least the recirculated air before the recirculated air flows back into the environment; characterised in that the regenerative treatment subsystem is located, in terms of air flow direction, in one of a: first location directly downstream of the environment, whereby there is an absence of a system component between the treatment subsystem and the environment to affect air flow, except for a means to allow air flow between the treatment subsystem and the environment; and second location directly upstream of the environment, whereby there is an absence of a system component between the treatment subsystem and the environment to affect air flow, except for a means to allow air flow between the treatment subsystem and the environment. first location directly downstream of the environment, whereby there is an absence of a system component between the treatment subsystem and the environment to affect air flow, except for a means to allow air flow between the treatment subsystem and the environment; and second location directly upstream of the environment, whereby there is an absence of a system component between the treatment subsystem and the environment to affect air flow, except for a means to allow air flow between the treatment subsystem and the environment. <EOS>