Sludge parameters Sludge treamentSludge parameters are the basis for the supervision and the construction of sludge treatment plants. They provide the enginieer with important notes over the existing (in-) organic portions, the settling behavior, the dewatering and the heat value of the sludge. • Total suspended solids TSS Mixed liquid from the reactor, the treated water, and the feed is filtered and heated in a 105°C oven. In this process, because of the low temperature, only the water is evaporated and nothing is burned. This is then weighed to conclude how much solids were in the feed, the treated water and the reactor. • Volatile suspended solid VSS The filtrate is taken from the 105 degree C oven and then placed in a 600 degree C oven where the organic materials burn and the left over inorganic materials are weighed. • Sludge volume index SVI Mixed liquid is taken from the reactor and taken to a slow mix to settle. After half an hour the sludge volume is measured. The SVI units are (ml/g).
Sludge parameters are the basis for the supervision and the construction of sludge treatmentplants. They provide the enginieer with important notes over the existing (in-) organicportions, the settling behavior, the dewatering and the heat value of the sludge.
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Total suspended solids TSSMixed liquid from the reactor, the treated water, and the feed is filtered and heated in a 105°Coven. In this process, because of the low temperature, only the water is evaporated and nothing isburned. This is then weighed to conclude how much solids were in the feed, the treated water andthe reactor.
• Volatile suspended solid VSSThe filtrate is taken from the 105 degree C oven and then placed in a 600 degree C oven where theorganic materials burn and the left over inorganic materials are weighed.
• Sludge volume index SVIMixed liquid is taken from the reactor and taken to a slow mix to settle. After half an hour the
sludge volume is measured. The SVI units are (ml/g).
• Stabilisation for a controlled degradation of organic ingredients and odourremoval
• Volume and weight reduction• Hygiene - the deadening of pathogen organisms• Ameliorating of sewage sludge characteristics for the further utilization or
disposal
The accompanying procedures are used to reach these goals.
The sludge consistency and appearance is changed through treatment. It must bepointed out that also the heating value of the sludge changes with the way of thetreatment. Anaerobic treated sludge has a lower heating value than raw sludge becauseof sewage gas production. This point has to receive attention by a thermal disposal.
StabilisationSearch :
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Sludge treament
Aerobic stabilisation can be performed simultaneously in an activated sludge plant whereby primary andsecondary sludges are continuously aerated for long periods of time. In aerobic digestion themicroorganisms extend into a respiration phase where materials previously stored by the cell are oxidized,resulting in a reduction of the biologically degradable organic matter. Thus, aerobic stabilisation of theentire excess sludge (including primary sludge ) is energy consuming. Additionally, it calls for extrareactor volume.Sludge digestion is carried out in the absence of free oxygen by anaerobic organisms. The facultative andanaerobic organisms break down the complex molecular structure of these solids setting free the "bound"water and obtaining oxygen and food for their growth. Anaerobic stabilisation processes work at normal
temperatures (< 40°C) or within the range of thermophile bacteria, where 50-65°C are reached alone bythe heat development of the biochemical processes. The chemical stabilisation of the sludge by means of wet oxidation or addition of quicklime and thermal stabilisation under high temperature and pressure, are
The anaerobic sludge digestion takes place in the highlyvisible digesters.
Digesters (source: klaerwerk-online.de)
ThickeningSearch :
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Sludge treament
A volume reduction of approximately 30 – 80 % can be reached with sludge thickening before a furthertreatment. At smaller wastewater treatment plants, where the sludge is driven off regularly, thickeningusually takes place directly in the sludge storage tank. The sludge is compressed at the tank bottom onlyby the force of gravity, while above the sludge a cloudy water layer is formed, which is taken off and ledback into the inlet.
On larger plants separate thickening basins exist. These basins are equipped with slow rotating verticalrods, which create micro canals in the sludge for a better dewatering. Also pure machine thickening isgaining more significance with e.g. non-stabilised sludges, that could rot during the storage.
DewateringSearch :
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Sludge treament
A further reduction of the sludge amount is mostly necessary after the thickening. The liquid sludge has to
be dewatered and has to conform to a dry and porous form. Dewatering can be done naturally (drybeds, solar drying), however this is only possible during a long period of time. Faster and smaller, but alsomore cost intensive, are machine processes such as pressing ( filter press ) and centrifugation(centrifuge).
For a good dewatering, size and firmness of the sludge agglomerates are important, so that these remainporous during the compression. Flocculants are often used to achieve as high as possible drying materialcontents at the machine dewatering and must be specifically co-ordinated with the accruing sludge.For the choice of the correct dewatering process it is important to consider a multiplicity of furtherboundary conditions: Quantity, structural situation, disposal, regulations, availability, personnel etc.
Sludge dryingSearch :
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Sludge treament
A further reduction of the sludge weight is possible with the help of sludge drying, by evaporating theremaining bound water in the sludge. However a significant reduction of the sludge volume cannot bereached with that method.
Sludge drying procedures are based particularly on contact-, convection or radiation procedures.Large amounts of air are not necessary during the contact drying, because the warmth is supplied by thecontact between the damp product and a heated wall. Only a minimum gas flow is often planned for theevacuation of steam. That has the advantage that the expenditure for exhaust air purification is small.
Convection drying obtains its effect by treating the sludge with hot-air. In addition ambient air is heated toa high temperature with a burner or steam heat exchanger and brought in contact with the sludge in adrum or belt dryer.
The obvious characteristic of radiation drying is that the warmth, which is necessary for the drying process,is supplied through radiation to the sludge. General examples for warmth supply through radiation aresolar radiation or infrared heating elements.
For the choosing the suitable drying method one should pay attention to
· Adherence to security· Environmental compatibility· Flexibility of the drying method in relation to variable sludge quantities
Because of their high investment and operating costs, sludge drying is mainly used in large wastewatertreatment plants.
The main characteristic of the activated sludge is the occurrence of microorganisms, which takeup solved food over their body surface or a cell mouth thereby contributing to wastewatercleaning. The biocoenosis of the activated sludge gives information over the condition of theactivated sludge and the cleaning achievement.
• BacteriaBacteria are simple, colourless, onecelled plants that use soluble food and are capable of self reproduction without sunlight. As decomposers they fill an indispensable ecological role of decayingorganic matter in stabilizing organic wastes in treatment plants. They are responsible for activatedsludge growing in domestic wastewater treatment. A wide variety of bacteria can be found in asludge flake.
A genus of common motile microorganisms(Spirobacteria) having the form of spiral-shapedfilaments.
Vitreoscilla
A genus of gram-negative, aerobic or microaerophilic,colorless filaments. It is motile by gliding. It is strictlyaerobic and produces a homodimeric bacterialhemoglobin, especially under oxygen-limited growthconditions.
Sphaerotilus
A sheathed filamentous bacterium that exhibits acharacteristic "false" branching. Once, incorrectly thoughtto be responsible for the majority of filamentous bulkingepisodes S. natans has now been recognized as onlyinfrequently encountered. Associated with nutrient
limitation, the organism is believed to not occur in plantswith anoxic zones.
Beggiatoa
Are large, colourless sulphur bacteria that include bothcolonial and filamentous forms, and they can dominatemicrobial communities associated with marine sediments.Beggiatoa spp. appears white due to the reflection of lightagainst their sulfur inclusions. Their size range in sizefrom a few millimeters to several meters.
Zoogloea
A colony or mass of bacteria imbedded in a viscousgelatinous substance. The zoogloea is characteristic of atransitory stage through which rapidly multiplying bacteriapass in the course of their evolution.
Beside bacteria, a number of species of protozoa such as flagellate-, ciliated- and amoebae protozoa havebeen identified in activated sludge . Protozoa are single-celled organisms that can consume food such asbacteria and particulate matter.
These organisms are partly on the activated sludge flakes. Other protozoa move lively on or between theactivated sludge flakes. The nematodes or rotifers are ranked among the multi-cellular organisms.
The sludge abstracted after the wastewatertreatment process contains in the unloaded stabilizedcondition: Carbon (50-70 %),
Hydrogen (6,5-7,3 %),Oxygen (21-24 %),Nitrogen (15-18 %),Phosphorus (1-1,5 %) and
Sulphur (0-2,4 %).
Water is the main component of sludge. Its amount depend on the sludge sort ( primary -, secondary ortertiary sludge) and the way of stabilisation (aerobic, anaerobic). Raw sludge has a water content of 93% to 99 %. Therefore a dewatering (up to approx. 35 % dry substance content) or drying (to over 85 %dry substance content) can be necessary for a further utilization.The second main component is the dry substance, which is made up of organic and inorganic substances.
Beside the main parts, sludge contains a large variety of trace components that have been separated fromthe wastewater. Organic and inorganic trace elements, which have its origin in wastewater, are foundenriched in the sludge.
Sludge sortsSearch :
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Sludge treament
Biological wastewater treatment produces different sorts of sludge within the individual processsteps. In the wastewater linguistic usage the following terms are used for sludge.
• Raw sludgeRaw sludge is untreated non-stabilized sludge, which can be taken from wastewater treatmentplants. It tends to acidify digestion and produces odour.
• Primary sludgePrimary sludge is produced through the mechanical wastewater treatment process. It occurs afterthe screen and the grit chamber and consists of unsolved wastewater contaminations. The sludgeamassing at the bottom of the primary sedimentation basin is also called primary sludge. Thecomposition of this sludge depends on the characteristics of the catchment area. Primary sludgeconsists to a high portion of organic matters, as faeces, vegetables, fruits, textiles, paper ect. Theconsistence is a thick fluid with a water percentage between 93 % and 97 %.
• Activated SludgeThe removal of dissolved organic matter andnutrients from the wastewater takes place inthe biological treatment step. It is done by theinteraction of different types of bacteria andmicroorganisms, which require oxygen to live,grow and multiply in order to consume theorganic matter. The resulting sludge from thisprocess is called activated sludge. Theactivated sludge exists normally in the form of flakes, which besides living and dead biomasscontain adsorbed, stored, as well as organicand mineral parts.The sedimentation behaviour of the activatedsludge flakes is from great importance for thefunction of the biological treatment. Theflakes must be well removable, so that the
biomass can be separated from the cleanedwastewater without problems and a requiredvolume of activated sludge can be pumpedback into the aerated part.
Biological aeration basin
Final clarifier
• Return activated sludgeThe activated sludge flows from the biologicalaeration basin into the final clarifier. Theactivated sludge flakes settle down to thebottom and can be separated from thecleaned wastewater. The main part of theseparated sludge, which is transported backto the aeration basin, is called return
activated sludge.
• Excess sludge, secondary sludgeTo reach a constant sludge age the unusedbiomass has to be removed from thebiological treatment system as excess sludge.The excess sludge contains not-hydrolysableparticulate materials and biomass due tometabolisms.
• Tertiary sludgeTertiary sludge is produced through further wastewater treatment steps e.g. by adding aflocculation agent.
• Bulking sludge, Floating sludge
• Digested sludgeDigested sludge accrues during the anaerobic digestion process. It has a black colour and smellsearthy. As a function of the stabilization degree anaerobic sludge exhibits an organic portion of thesolid from 45 to 60 %.
Sludge sorts
Copious growth of filamentousorganisms - problems and solutions
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Sludge Treatment
Many European wastewater treatment plants have problems in the stage of biologicaltreatment, which are related to copious growth of filamentous microorganisms.
Reasons for the massive appearance of filamentous microorganisms
- Little sludge load- Lopsided composition of wastewater- Variations in the wastewater
Due to the increasing demands concerning the purification achievement the sludge load, i.e. theratio of the daily fed BOD load and the dry solid matter in the activated sludge tank, decreases.
This leads to copious growth of filamentous organisms, because these organisms attain - unlikethe floc-forming bacteria – high growth rates even if substrate and oxygen concentrations arelow.
Another reason for the dominance of filamentous microorganisms is the lopsided composition of wastewater, as it occurs in many industrial firms. But also variations in the effluent; e.g.unstable flow, varying temperature and changes in the composition of nutrients in thewastewater, can avail the growth of these organisms, because they are in general much moreunassuming than the floc-forming organisms.
Symptoms that are related to copious growth of filamentous microorganisms
When talking aboutfilamentous organisms,usually bacteria are meant, insome cases also filamentousfungi. Having a certainamount of filamentousorganisms is advantageous,because in comparison to thefloc-forming bacteria theyachieve more effectivenutrient uptake. Furthermoretheir prolate build supportstheir feature to catch floatingparticles. These advantagesfront the disadvantages of alower sludge settleability,which increases the costs of sludge treatment drastically.
An increased development of filamentous microorganismscauses two extremelyundesirable phenomena:
- Bulking sludge
- Floating sludge
Abb. 1 : Sickly sludge with high share of filamentous microorganisms Bulking sludge
The term bulking sludge refers to sludge with extremely bad settling andthickening characteristics. In most cases bulking sludge accumulates in theclarifier, where it forms a thick layer and has to be removed to prevent that itflows through the outlet and debits the on-site preflooder.
The settleability is described by the parameters of sludge volume (SV) andsludge volume index (SVI). The sludge volume is the specific volume, whichsludge takes up in a certain time of settling (mostly 30 min in a barrel) in ml/l.The sludge volume index describes the volume that 1 g sludge (referring to drysolid matter) has after 30 min. of settling. For calculating the VSI the VS in ml/lhas to be divided by the dry solid matter (TS) in mg/l.
VSI = VS (ml/l) / TS (g/l) = (ml/g)
The sludge volume index of bulking sludge is more than 150 ml/g.
Floating Sludge This is the second phenomenon that occurs in sickly activated sludge tanks. Floatingsludge floats on the surface due to the copious growth of actinomycetes and certainother filamentous organisms, which have a hydrophobic cell surface. Thehydrophobic cell surface adsorbs air and nitrogen gas bubbles and causes the sludgeto swim upwards. Floating sludge should be removed quickly, because it also leads to
the formation of foam in the septic tanks of anaerobic sludge treatment.
There are several methods to contain and prohibit copious growth of
filamentous microorganisms. A technical solution for preventing bulking sludgeis the application of high-loaded tanks (selectors) as a stage before the
activated sludge tank or the installation of cascade-systems. In both cases the
occurring substrate gradient compensates the low loading of the wastewater
and bulking sludge can be prevented. A similar method of solution is found in
the stage of biological phosphate removal in advanced wastewater treatment:
Here the effect of the upstream, anaerobic mixing tanks is the same as of the
selectors.
Other methods to achieve a better settleability is bypassing the preliminary
sedimentation, improving the wastewater characteristics of lopsided effluents or
the addition of precipitants and flocculants.
An easier and less dramatic measure than adding constructive selectors or
flocculants is supplying LennSludge .
Mechanical Flotation
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This process of mechanical dispersion of air bubbles 0.2 to 2 mm in diameter is mainly used for the separationand differential concentration of ore pulp. It ‘s possible to add a collector agent and an activator or a depressor inthe water.
In treating oily water from oil refineries, the separation of oil by mechanical flotation is called froth flotation. It isachieved by adding an organic coagulant or demulsifying agent. Flotation units are constructed with three or four cells in series.
Froth flotation by injection of air may also be used for the separation of surfactants.
Flotation
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As opposed to settling, flotation is a solids-liquid or liquid-liquid separation procedure which is applied toparticles whose the density is lower than that of the liquid they are in.
There are three types of flotation: natural , aided and induced flotation .
Natural flotation Aided flotation Induced flotationValid if the difference indensity is naturally sufficientfor separation.
Occurs when external meansare used to promote theseparation of particles thatare naturally floatable.
It occurs when the density of particles is artificiallydecrease to allow particles tofloat. This is based on thecapacity for certain solid andliquid particles to link up withgas (usually air) bubble toform « particle-gas » with adensity lower than the liquid.
DAF (Dissolved air flotation) is a procedure of induced flotation with very fine air bubble or« microbubbles », 40 to 70 microns.
Mechanical flotation is a term used in the mineral industry to describe the use of dispersed air to producebubble that measure 0.2 to 2 mm in diameter.
The conditions for the application of the different flotation processes in water treatment are
Contact us Clarification consists in removing all kind of particles, sediments, oil, natural organic matter anc colourfrom the water to make it clear.
A clarification step is the first part of conventional treatment for waste andsurface water treatment.It usually consist in:
- Screening- Physical chemical treatment is a generic term for Coagulation -Flocculation- Sedimentation or Flotation , upon particles properties and water type- Fine filtration
LIME-SODA ASH WATER TREATMENT METHOD
Lime-soda ash treatment for the reduction of hardness involves the addition of slake lime[Ca(OH) 2] to a hard water supply to remove the carbonate hardness by precipitation withthe precipitation being removed by filtration. Non-carbonate hardness is in turn reduced
by the addition of soda ash (Na 2C0 3) to form insoluble precipitate which is also removedby filtration.
This particular method of removing hardness a sometimes used by municipal waterplants to reduce the amount of calcium and magnesium in a water supply. While it isquite effective in reducing hardness, it is not a complete removal treatment.
