Filaments and Bacteria...
Volume - 6...

Glossary of Terms...

ACTIVATED SLUDGE : A flocculent microbial mass, produced when sewage is continuously aerated.
BIOCHEMICAL OXYGEN DEMAND (BOD) :The amount of dissolved oxygen consumed in the oxidation of biodegradable organic matter by microbiological action when a sample is incubated, usually for 5 days at 200 ^oC. It is often referred to as BOD₅.
BULKING SLUDGE :A phenomenon, which occurs in, activated sludge plants when sludge/biomass occupies an excessive volume and does not settle readily so that in extreme cases the effluent from the final settlement tank contains an excessive amount of suspended solids. Usually associated with the presence and predominance of filamentous micro-organisms in the biomass. Appendix I explains bulking sludge in more detail.
COD :The amount of oxygen consumed from a specific oxidizing agent in the oxidation of the polluting matter present in a sample. As normally determined i.e. as from the silver catalyzed dichromate, it approximates to the oxygen theoretically required for complete oxidation of the carbonaceous matter in the sample to carbon dioxide and water.
DEFLOCCULATION :Flocs disperse as :
• Sludge ceases to react with waste.
• Protozoa cease activity and allow single cell bacteria etc upon which they feed to multiply.
• Bacteria in the floc die and floc breaks up.
• Excessive oxygenation.
• Shock load.
• Low pH.
• Low dissolved oxygen.
DISSOLVED AIR FLOTATION (DAF) :A process that can be used for oil and grease removal, for concentration of activated sludge, or as a pre-treatment in physical – chemical methods of treatment. In the concentration of activated sludge, flotation is achieved by the attachment of microscopic sized air bubbles to the activated sludge flocs thereby reducing their specific gravity and causing them to rise to the surface where the concentrated activated can be skimmed off.
DENITRIFICATION :The reduction by microbial or other of Nitrate to Nitrogen gas, or in some cases to Nitrous oxide.
DISSOLVED OXYGEN :Oxygen dissolved in a liquid, the solubility depending on the temperature, partial pressure and salinity, expressed in mg/l.
FLOC FORMERS / FLOCCULATION :Micro-organisms in activated sludge usually agglomerate with each other and with suspended solids into clumps. This is called flocculation by biological action. Individual clumps are termed flocs. This phenomenon may be due to :
• Free cells becoming embedded in a gelatinous mass.
• Bacteria, which grow as free cells, flocculate under certain conditions.
• Bacteria adhering to extra cellular material produced by fungi.
• Bacteria being flocculated by protozoa.
• Possibly all of the above plus some other mechanism.
• Flocculation can also be achieved by the addition of a chemical or by stirring the waste-water by mechanical means.
FOAMING :The formation of a foam or froth on the surface of :
• The mixed liquor in an aeration tank.
• The sludge in an anaerobic digestion tank.
• A body of water, caused by detergents, surfactants and/or polyglycols lowering the surface tension of the liquid.
Foaming can also be caused microbiologically by certain filamentous bacteria particularly actinomycetes e.g. Nocardia.
F/M RATIO :Ratio of Food to Mass ( Biomass ). In activated sludge process, it is the ratio of loading in kg of BOD/day to weigh in kg of either volatile or total suspended solids in the mixed liquor. At start up of an aeration basin, the concentration of the biomass is low while nutrient concentration is high. Thus the F/M is high. During the growth of biomass, cells multiply and compete for the available food/nutrients. As a result the food/nutrients supply decreases and the F/M ratio becomes much lower a low F/M.
F/M = BOD of Sewage ( kg/m³ ) x Influent Flow ( m³ /d ) : Reactor volume ( m³ ) x Reactor Solids ( MLSS ) ( kg/m³ )
F/M VALUES FOR ACTIVATED SLUDGE SYSTEMS :
High Rate : 0.6 – 1.8 kg BOD/kg MLSS.day
Conventional : 0.1 - 0.6 kg BOD/kg MLSS.day
Low Rate : 0.05 – 0.2 kg BOD/kg MLSS.day
HIGH RATE :Any plant which is operated purposely at a hydraulic or organic loading significantly greater than that usually employed, without special regard to the quality effluent which the plant produces or to the type of waste being treated.
- High Rate Filter : A biological filter containing a coarse medium or synthetic medium operating with a hydraulic load exceeding 3 m³/m³.day or an organic load exceeding 2 kg of BOD/m³.day. Also called a High Rate Tower.
- High Rate Activated Sludge Process : A modification of the activated sludge process whereby a much shorter aeration period than usual and possibly a high mixed-liquor suspended solids concentration are used.
HYDRAULIC LOADING :Also termed Volumetric Loading. When applied to biological filters e.g. percolating filter, it is the rate of application expressed in cubic meters of wastewater per cubic meter of media per day.
Hydraulic Loading = Flow rate ( m³/day ) : Media volume ( m³ )
The normal loading rate for this application is 0.65 m³/m³.day. When applied to biological filters e.g. High Rate Tower, it is the rate application expressed in cubic meters of wastewater per surface area ( m² ) of media per hour.
Hydraulic Loading = Flow rate (m³/hr ) : Surface area of media ( m² )
The normal loading rate for this application is 1.5 m³/m².hr.

MIXED LIQUOR :A Mixture of wastewater and activated sludge undergoing aeration and circulation in an aeration basin or channel of an activated sludge plant.
MIXED LIQUOR SUSPENDED SOLIDS ( MLSS ) :The weight of dry solids in mg/l of mixed liquor in the aeration tank or channels of an activated sludge plant.
Determination of MLSS : Filter a know volume of mixed liquor through an Buchner Funnel onto predried and preweighed filter paper e.g. GFC. Dry at 103 ^oC to a constant weight ( usually one hour ) and reweigh :
Result : MLSS = mg/l
Typical MLSS values for the main activated sludge systems are as follows :
MLSS ( mg/l )
High rate : 4,000 - 5,000
Conventional : 1,200 - 3,500
Nitrifying : 3,500 - 5,000
VITOX : 3,000 - 12,000
Oxidation ditch : 3,500 - 5,000
MIXED LIQUOR VOLATILE SUSPENDED SOLIDS ( MLVSS ) :This refers to mixed liquor solids, which volatilise in 20 minutes at 600 ^oC. The solids, which remain after ignition, are referred to as ash. MLVSS are usually expressed as a % of MLSS.
NITRIFICATION :The oxidation of Ammonia to Nitrite and Nitrate . Usually carried out by autotrophs , which obtain their energy from the oxidation reaction.
ORGANIC LOAD :The organic load is expressed as the kg of BOD per unit volume of aeration basin / filter medium. It is a measure of the amount of BOD, which is applied per unit volume of aeration tank capacity or per unit volume of filter medium.
Organic Load = Influent Flow ( m³/day ) x BOD of Sewage ( kg/m³ ) : Reactor Volume ( m³ )
Typical organic load values for a Fixed Film ( Biological Filter ) are as follows :
Organic Load
High Rate : 0.32 – 1.0 kg BOD/m³ media.day
Intermediate Rate : 0.04 – 0.48 kg BOD/m³ media.day
Low Rate : 0.08 – 0.32 kg BOD/m³ media.day
In general a BOD loading in excess of 0.5 kg BOD/m³.day is High Rate. The organic load will vary over a day with fluctuation in both influent flow rate and strength. Most accurate values ( BOD ) will be obtained from a composite sample, of samples of equal volume taken at 1 hour intervals over a 24 hour period.
OXIDATION DITCH :A method of 6treating crude or settled wastewater in witch two parallel channels are joined at the ends to form a closed circuit. It can be equipped with one or two aeration rotors, used for aerating a mixture of the wastewater and activated sludge prior to separation in a settlement tank. ( First introduced in Holland in 1958 ).
PIN POINT FLOCS :Small particles remaining suspended after sludge has settled. In the process, pin point flocs pass out in the effluent. Two types :
• Grey, fly ash type, no BOD, inert.
• Pink/Brown, doesn’t rise/settle, has high BOD.
PERCOLATING FILTER :A bed of material relatively inert to natural processes of degradation ( such as slag, molded plastics, clinker etc. ) usually contained within circular or rectangular walls and so constructed that air is continuously present throughout the bed. In use, settled sewage is distributed uniformly over the upper surface and trickles through the bed to underdrains, thus giving rise to the development on the packing material of a biological film containing aerobic bacteria and fungi which about oxidation and clarification of the sewage. Essentially, the process does not rely on filtration except at the microscopic level and the bed is incorrectly described as a filter. The first as above were installed at Salford in 1897.
RETENTION TIME :The period during which wastewater is retained within a unit. This is based on :
• Maximum Flow
• Average Flow or
• Dry weather Flow.
Retention Time = Reactor Volume ( m³ ) : Total Daily Flow ( m³/day )
This is the amount of time which sewage undergoes aeration in the reactor. Also knows as Hydraulic Retention Time. Usually 6-10 hours.
RETURN ACTIVATED SLUDGE :That portion of the activated sludge separated from the mixed liquor in the secondary settlement tanks, which is returned to the aeration tanks. Normally expressed as RAS.
RETURN SLUDGE RATE :25% of influent is the normal rate. Sometimes reasserting sludge before mixing with more wastewater improves settlement.
ROTATING BIOLOGICAL CONTACTOR :This is a fixed film type of effluent treatment system. A unit consisting of a series of closely spaced, parallel discs are mounted on a shaft, which is supported just above the surface of the wastewater to be treated. The lower parts of the discs therefore extend to the wastewater, as the discs are slowly rotated. A biological film or slime layer develops on the wetted surfaces as the are alternatively submerged, exposed to nutrients and raised out of the liquid to oxidize the absorbed nutrients.
SELECTOR : Selected systems are more resistant to low F/M bulking.
• A selector is a mixing basin/channel where the RAS and influent wastewater mix before entering the aeration basin.
• Selectors can be aerobic, anaerobic ( DO, Nitrate absent ), anoxic ( DO absent, Nitrate present).
• Selectors should be designed to remove virtually all the soluble BOD in the wastewater. To do this selectors can be divided up into compartments.
• Sludge developed in successive compartments exhibit several important characteristics. Soluble BOD₅ uptake and where appropriate, oxygen uptake rate is more rapid than for sludge’s developed in a non selector completely mixed system.
• The aim of the selector is to provide a short term, high substrate condition that favours certain floc formers, but discourages filaments.
• Hydraulic retention time in a selector varies from 30 minutes to about 2.0 hours depending on whether it is an aerobic or anaerobic selector.
SETTLED VOLUME ( SV ) :Volume occupied by sludge after settling. Normal the 30 minutes Settled Volume is determined. Place 1 liter of mixed liquor in a clean ( glass ) graduated cylinder, sit for 30 minutes and read volume of sludge settled from graduations on cylinder.
Result : ml of sludge per liter of mixed liquor.
SLUDGE AGE :Number of days over which total mass of sludge wasted mass of sludge undergoing aeration when an activated sludge plan is operating in equilibrium. Sludge age ( days ) is calculated as follows :
Volume of liquid in aeration tank ( m³ ) x MLSS in aeration basin ( mg/l ) : {Sludge waste rate ( m³/day ) x MLSS of waste sludge ( mg/l )} + {Effluent discharge rate ( m³/day ) x TSS in final effluent ( mg/l )}
Note : Effluent discharge rate = Influent flow rate.
Typical sludge age values for the main activated sludge systems are as follows :
Sludge Age ( days )
High rate : 1 - 3
Conventional : 3 - 8
Nitrifying : 8 - 12
VITOX : 5 - 25
Oxidation ditch : 60 - 90
Reciprocal of Sludge Age = Specific growth rate of sludge.
SLUDGE DENSITY INDEX ( SDI ) :Prepared by Donaldson in the USA. It is a measure of the settleability of activated sludge. It is the weight of dry solids contained in 100 ml of sludge settled for 30 minutes as for SVI. The results are expressed as g/100 ml.
SDI = MLSS (%) x 100 : Settled volume of sludge (%) after 30 min
Good Settling = 2.0 g/100 ml, poor Settling = 0.3 g/100 ml
It is the reciprocal of SVI x 100.
SLUDGE LOADING :Weight ( quantity ) of BOD applied to an activated sludge plant per day per unit weight of activated sludge undergoing aeration. Same as F/M.
SLUDGE VOLUME INDEX ( SVI ) :Is the volume ( ml ) occupied 1 gram of activated sludge after settling aeration basin mixed liquor for 30 minutes in a 1 litre graduated cylinder. It is used for day to day monitoring of settlement in an activated sludge plant. SVI is calculated by dividing the 30 minutes Settle Volume ( SV ) by Mixed Liquor Suspended Solids ( MLSS ). Units are ml/g. It was described by Mohlman in 1934. It is sometimes to as the Mohlman Index. e.g. – for a MLSS of 3,000 mg/l ( 3.0 g/l ).
STIRRED SPECIFIC VOLUME INDEX ( SSVI ) :This is a more accurate measure of sludge settleability. The settleability of a sludge may be monitored using SSVI at an MLSS concentration of 3.5 g/l.
SSVI = *Height of sludge ( cm ) x 100 : Initial height of sludge ( cm ) x MLSS ( g/l )
( * : Height after 30 minutes )
Sludge from an average activated sludge plant operating well should have an SSVI of less than 100 ml/g. Sludge with very good settling characteristics will have an SSVI of 40-50 ml/g. Sludge with poor settling will have SSVI in excess of 120 ml/g and very poor settling will be observed with an SSVI greater than 250 ml/g. This test utilizes a 10 cm WRC settling apparatus. To follow are normal SSVI values for the main types of activated sludge systems :
SSVI ( mg/l )
High rate : 120 - 140
Conventional : 40 - 100
Nitrifying : 40 - 80
VITOX : 40 - 70
Oxidation ditch : 100 - 200
UNOX :This is an activated sludge process recently develop in the USA, in which a surface aerator is used for oxygenation the mixed liquor in an oxygen enriched atmosphere within an enclosed tank.
VITOX :The VITOX system utilizes pure oxygen to mix the mixed liquor in the aeration basin and to provide a source of oxygen for bio-oxidation. The VITOX system was originally intended as a method of uprating the oxygenation capacity of an overloaded sewage treatment plant. In addition it has proved very effective in the treatment of high strength industrial wastes. Many operators of this system report very low sludge yields. More recently the potential of VITOX units as the first stage activated sludge process has been realized.