Biomass selection and control for continuous flow granular/flocculent activated sludge processes
A continuous flow granular/flocculent sludge wastewater process selects for granule biomass capable of nitrogen and phosphorus removal and controls granule size and concentration of granular and flocculent sludge for optimal nutrient, organic, and solids removal in a smaller footprint. A series of biological process zones lead to a secondary clarifier. Mixed liquor sludge, preferably from an aerobic zone, goes through a classifier or separator processing flow from the aerobic zone, to the secondary clarifier. In a sidestream process that can be included a portion of sludge preferably from an aerobic zone goes through a classifier or separator to selectively produce a granular-rich effluent, and the clarifier may also have a separator to further concentrate granular biomass, most of which is cycled back to an initial multi-stage anaerobic process zone. The anaerobic zone is structured and operated to encourage growth of granules in subsequent process zones.
1. A wastewater treatment process with a series of process zones including biological process zones, with continuous flow of influent wastewater through the series of zones, a first of the biological process zones being an anaerobic zone for encouraging selective growth of granular biomass, comprising:
providing at least two successive stages in the anaerobic zone, including a first stage and a second stage,
into the continuous flow of influent wastewater, introducing a first portion of a recycled activated sludge (RAS) stream from a downstream process zone of the series of process zones to produce a continuous combined raw influent/RAS flow into the first stage,
the first stage being of preselected volume, and dwell time in the first stage being no greater than 30 minutes, promoting contact between the influent wastewater, with food, and the RAS, with biological activity, such that granular biomass present in the RAS is exposed to the food of the influent wastewater for consumption and storage leading to growth of granules in further process zones,
operating a mixer in the first stage to mix the RAS and incoming wastewater to provide a mixed liquor,
moving the mixed liquor in continuous flow to the second stage,
introducing to a bottom region of the second stage a second, remaining portion of the RAS to mix with the mixed liquor in the second stage,
in the second stage, operating a mixer intermittently such that mixing of the incoming RAS and sludge from the first stage occurs for a time, then the mixer is shut off for a selected period of time whereby heavier granular biomass tends to settle deeper than floc biomass in the second stage, so that said remaining portion of RAS entering the second stage comes into intimate contact with the settled granular biomass and particulate BOD, and
the proportions of incoming wastewater and RAS in the first stage being such as to create a food to mass ratio (F/M) of at least 5.
2. The process of claim 1 , wherein the second, remaining portion of RAS is introduced into the second stage using a distributor that distributes the incoming RAS essentially uniformly across the width of the second stage and at said bottom region of the second stage.
3. The process of claim 1 , including a third stage downstream of the second stage, mixed liquor being moved continuously from the second stage to the third stage.
4. The process of claim 3 , including operating a mixer intermittently in the third stage.
5. The process of claim 1 , wherein the series of process zones includes at least an aerobic process zone positioned downstream of the anaerobic zone, and including a granular/floc separator downstream of the aerobic process zone, and a clarifier connected to the separator such that the clarifier tends to concentrate granular-rich sludge in a bottom area of the clarifier, with at least a portion of the granular-rich sludge being directed back to the first stage of the anaerobic zone as said RAS.
6. The process of claim 5 , wherein the series of biological process zones includes an anoxic zone between the anaerobic and aerobic zones.
7. The process zone of claim 1 , wherein said first portion of the RAS is 10% to 50% of the RAS.
8. The process of claim 1 , wherein said first portion of the RAS is 25% to 40% of the RAS.