404 volume 410 of the reservoir is typically void of polymeric container closures porous barrier 408 may be used when the polymeric container closures are less dense than water and a static condition is desired biological filtration systems 400 and 420 may be open to air e g at the top of the reservoir and may include one or more fluid inlets and one or more fluid outlets as depicted in fig4 a and 4b biological filtration systems 400 and 420 include fluid inlets 412 and 414 and fluid outlets 416 and 418 the placement fluid inlets and outlets is not limited by the depiction in fig4 a and 4b one or more fluids may be provided to reservoir 404 of biological filtration systems 400 and 420 via fluid inlet 412 414 or both when two or more fluids are provided to biological filtration system 400 one of the fluids may be a gas in some cases a biological filtration system includes a pump coupled to one or more of the fluid inlets and fluid outlets fig4 c depicts biological filtration system 440 with pump 442 fluidically coupled to reservoir 404 and basin 444 pump 442 provides aqueous liquid e g wastewater or other water to be treated from basin 444 to reservoir 404 via inlet 412 treated aqueous liquid may be drained from reservoir 404 via fluid outlet 418 in some cases e g when reservoir 404 is open to the atmosphere biological filtration system 440 includes fluid inlet 412 and fluid outlet 418 to the exclusion of other inlets and outlets in other cases biological filtration system 440 includes one or more additional fluid inlets and or one or more additional fluid outlets such as fluid inlet 414 and fluid outlet 416 in one implementation biological filtration system.