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Activated Sludge


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Activated sludge systems

Very roughly, undesirable/excess wastewater constituents may be present in either or both of two forms, namely soluble and/or particulate, a cutting point being about 0.45 µm. To a certain extent, nonsoluble fractions can or ought to be removed by appealing to essentially physical pretreatment means, e.g. settling, flotation, screening. However, this first step leaves us with an at times sizable amount of material in soluble form that will still have to be dealt with. Because further appeals to physical processes will be of no avail (also law of diminishing returns), present environmental engineering practice brings in and banks on the phenomenal power of natural/biological processes. Wastewater treatment plants essentially replicate in somewhat controlled mode what has been going on in nature for ages: biological processes. In this way, "troublesome" soluble components are gladly gobbled up by living matter in specially conditioned "microorganism farms," literally sizable "swimming pools full of bugs." Typical suspended-growth embodiments include aerated lagoons and activated sludge basins. Common to all approaches is growth and development of large microorganism inventories that will pick up soluble stream components, be it organic matter, nutrients like N and P, which in turn can and will be subsequently removed from said stream by ... physical means, e.g. settling, flotation, screening/membranes.


     The volatile solids content of the mixed liquor in an activated sludge process typically is considered to represent the mass of microorganisms that is available for waste treatment.  However, not all volatile solids are active microorganisms.  Inert cell matter and volatile but nonbiodegradable influent solids also contribute to the mixed liquor volatile solids.  Oxygen uptake occurs when active microorganisms consume biodegradable constituents of the wastewater.  Respirometer-aided methods can provide oxygen uptake measurements which can be used to estimate actual active biomass in mixed liquors.  Case studies show that mixed liquor volatile solids typically range from 5 to 25% active microorganisms.

     We can't go much higher in MLVSS than about 2,000 mg/L and still get good oxygen transfer in the aeration basins. 


     In activated sludge processes, the oxygen uptake rate (OUR) of the mixed liquor is frequently used to assess the activity of the active biomass, to determine the response of the mixed liquor organisms to changes in wastewater compositions such as may be caused by variations in industrial wastewater production, and to identify the presence of toxic inputs that can adversely affect wastewater treatment processes.  Typically these measurements are made dissolved oxygen depletion in conventional 300-ml BOD bottles.  However, there is considerable more value in measuring oxygen uptake for longer times using respirometers that operate on a continuous or semi-continuous basis.  Such measurements provide significant insight into the biodegradation characteristics of individual or specific wastewater constituents in the process being monitored.  Oxygen uptake reactions can be used to identify and sort out numbers of biodegradation reactions present in activated sludge processes.

     Oxygen consumption in activated sludge processes is expressed in terms of either oxygen uptake rate (OUR),  mass of oxygen per liter of mixed liquor per hour, say mg O2/L-hr, or specific oxygen uptake rate (SOUR) where oxygen consumption is expressed as mass of oxygen per unit of microorganism mass per hour, say mg O2/g MLVSS/hr.  Oxygen uptake rate is related to substrate removal, measured as COD or less desirably BOD, and biomass respiration.  Substrate OUR is related primarily to the amount of COD removed and biodegradation kinetics.  Respiration oxygen uptake is related primarily to the MLVSS concentration and the associated SRT.


In most cases the designer will maintain constant DO levels so as to make sure biological needs are met.  Target DO levels are typically set slightly above minimum required to insure aerobic conditions, provide for fluctuations and avoid development of undesirable organism types.  Higher than required DO levels translate into wasted energy; oxygen deficits may result in poor sludge settling characteristics, foaming and undesirable mixed liquor organism predominance.


     For conventional activated sludge of average rate, i.e. medium rate, it is generally recommended approximately 50 lbBOD/1,000 cu.ft. as maximum.  For process stability and better assurances of performance, [fine pore/fine bubble] diffused aeration systems favor the use of low f/m and that is generally restricted to about 10-15 lbBOD/1,000 cu.ft.  This significantly lower rate sizing tip takes into account process recommendations (extended aeration) as well as diffuser technology old hands recommendations.


     Improperly designed/operated activated sludge installations frequently reflect very high F/M ratios.  Very high F/M values means the organisms are very active and do not settle well in the clarifier.  If they do not settle well two problems arise, i.e. 1: one cannot return enough solids to the system to maintain the process and 2. high levels of BOD and suspended solids are discharged which prevents one from achieving effluent limits.  Fouling can also translate into complete wwtp collapse.  High F/M concentrations also generate substantial levels of suspended solids at very low concentrations in the clarifier which means there are huge volumes of sludge to handle.



     In order to maintain required mixed liquor suspended solids (MLSS) for an activated sludge process, a certain amount of excess or waste activated sludge needs to be removed from the system, say a given fraction from the system's clarifier underflow.



     Design of the aeration tanks is also important for optimum efficiency.  Proper tank design can make it or break it for any given aeration system selection. It is often the case that the same identical piece of equipment or given amount of hardware will deliver far more return for the investment if only careful/generous process considerations are taken into account, adding buffer treatment capacity.

     For best performance, surface aerator vendors frequently suggest recommended/ minimum/maximum liquid depths for their standard units. If basin is too deep, the aerator may not be able to effectively pump up beyond a given depth thus resulting in idle pockets or even whole layers, at least as regards intended aerobic activity. 

     As surface aerators pump up and splash out liquid, they induce very high velocity flows directly beneath them, and in some instances may cause damage to basin floor. In earthen or lined basins, aerator vendors usually recommend the use of bottom concrete pads directly below the units, although wastewater and concrete/other proposed material compatibility must be verified. 

     Minimum depth manufacturer's specs for 1 and 2 Hp aerators can be say 1.2 meters (4ft. depth).

     Minimum depth say for larger 10 and 15 hp units can be say 3m (10ft. depth).

     In order to be able to use them in [even] shallower basin anti-erosion assemblies must be incorporated.

     Surface aerators also project sizable horizontal patterns which may unduly impinge on basin walls risking leaking potentials. Riprapping or similar shore structures can protect basin walls.

     Fine bubble diffusers can work say at 2.5m water depth but deeper basins will give greater efficiency and superior results on capital costs.  Diffusers are directly dependent on liquid depth for their aggregate efficiency.  As a result, if the basin depth is doubled, it will approximately use the same horsepower but it will take only roughly one-half the number of required diffusers, i.e. capital cost of the diffusers is about 50%.  Using deeper basins, e.g. 5m, and larger volumes offer much better assurances of performance.  It must be said that one of the authors once witnessed a probably still existing wastewater treatment plant at an edible oil plant having a detention time of ... [only] ten (10) minutes.

     The reason the horsepower stays the same is that the pressure increase offsets any SOTE increase you make the basin and diffusers deeper.



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