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Practice English Speaking&Listening with: Lecture 29 UASB (Contd) Sludge Treatment

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Last two classes we were discussing about anaerobic process. We have seen what is the

fundamental of anaerobic process and what all are the parameters or the environmental

conditions required for the proper

functioning of anaerobic process and why the anaerobic process was not becoming popular

in the recent past. We have also seen in detail about the high rate anaerobic processors or

anaerobic reactors

commonly used nowadays which can retain last quantities of biomass inside the system.

We were discussing about UASB reactor the Up flow Anaerobic Sludge Blanket reactor which

is the one most commonly used for domestic wastewater treatment especially in tropical

countries. We have seen

how to design the reactor, how to design the settling zone, how to design the gas, liquids,

and solid suppression systems etc in detail. Now we will discuss about the design of inlet

as well as the outlet

arrangements in the UASB reactor and further we will discuss about how the sludge coming

from the biological treatment systems as well as chemical treatment systems can be handled.

This is the picture or figure we were discussing about in detail last class. This is the schematic

of an Up flow Anaerobic Sludge Blanket reactor. We have seen how it is functioning. The wastewater

is coming like

this and the influent is getting equally distributed in the bottom of the reactor and the biological

sludge which is flocculent in nature will be in the bottom of the reactor and as the

wastewater passes through that

one will pass through this sludge which is nothing but the active biomass. So the biomass

will be in contact with the organic matter present in the wastewater so it will be undergoing

degradation. As a result new

biomass will be generated and the biomass will be coming up as well as the biogas whatever

is generated as the byproduct of microbial degradation also will be coming up but if

the solids escape along with the

liquid then the efficiency of the system will be coming down. So we are providing a specific

system here including this deflector beam and this corn arrangement and this is known

as Gas Liquid Solids

Separator.

We have seen in detail how to design this one. the basic objective of this GLSS is to

separate solid phase, liquid phase and gas phase so that all the biomass will be retained

in these digesters and the biogas will

be collected here and used for other purposes so we get a completely treated clear effluent

here which can be discharged without any problem. This will be meeting the effluents standard.

We have discussed in detail about the digestion zone design, settling zone design and GLSS

design. Now we will see how to design the influent distribution device. The distribution

of wastewater in the reactor is

very, very important because if uniform distribution is not there then the contact of organic matter

and the microorganism will not be proper so the efficiency of the system can come down.

If the distribution is

not uniform short circulating can take place in the system so only a very little amount

of organic matter will be coming in contact with the active biomass so naturally that

treatment efficiency will be coming down.

So, for the proper functioning of an anaerobic reactor or UASB reactor the uniformed distribution

is essential, this is done using flow splitting devices. The recommended value is that at

least one inlet should be

there for every one to two meter squared of the reactor area. Why this uniform distribution

is essential or how can we guarantee the uniform distribution. Thus, we have to verify these

points or we have to take

care of the following things. One should verify the flow at each inlet point.

Though we provide a proper distribution system if choking takes place in certain points then

the distribution of the wastewater in the reactor zone will not be proper so definitely

there will be short circuiting and

some zones will not be getting any organic matter so that will be advisably effecting

the reactor performance so verification of flow at each inlet point is very very essential

and obstruction of inlet point should be

detectable and if obstruction is found it should be removed. These are the important

points to be taken care to get a uniformed distribution in the reactor.

This shows a distribution system. This is the way the water is coming here and this

is the distribution well it is going to different points. As you can see this one from here

it is coming to each inlet point and the

main influent is coming like this and this is the rear and from here it is getting distributed

to various inlet points and it will be going down. This inlet either it can be from the

bottom of the reactor using a nozzle

and the nozzle opening will be in the bottom of the pipe the reason is if the nozzle is

facing to the sludge then the sludge will be entering the nozzle opening and the nozzle

will be getting clogged.

So usually what will happen is a ball type or bell type arrangement will be there in

inlet of the inlet tube and that will be covering or that will be providing the tube getting

clogged because of the sludge and the

wastewater will be entering from both sides and it will be going up so it will provide

a proper distribution of the wastewater.

In some other cases what is usually done is the inlet will be coming from the top of the

reactor straight to the bottom so because of the hydraulic pressure it will be getting

distributed so choking can be avoided

in that case. This is the arrangement usually used in circular devices and this is a rectangular

inlet device. And the effluent collection devices are also very, very important. The

effluent collection devices usually

employed in UASB reactor are very similar to the wall which we use in sedimentation

tanks.

We have Gutters with V notches to collect the effluent at the top of a UASB reactor.

Here we can see the Gutters with V notches so it will be getting collected here and the

effluent is coming from here and it will

be going either for treatment or for disposal. So this is the alternate design for effluent

collection systems. So whatever we have designed or we have discussed in detail of the sedimentation

tank is the same type.

We know that in the UASB reactor the top portion will be acting as a sedimentation tank so

the same type of an effluent collection device can be used for the USAB reactors.

We were discussing till now about the anaerobic treatment systems. But these anaerobic treatment

systems in most of the cases cannot be used as a single treatment process for meeting

the effluent discharge

standards. The reason is anaerobic process will not be able to take care of the pathogens

whatever is present in the wastewater or the microbial concentration whatever is present

in the wastewater will be getting

reduce only in one log order or two log order so we have to have a proper post treatment

unit to take care of these things.

Moreover, in UASB reactor as we know the substrate is not getting oxidized and the biomass synthesis

is very very less we have seen that the organic matter whatever is entering in the system

only 8% is going

for anabolism or the sludge generated in anaerobic process is very very less compared to the

aerobic process. We know that for the microbial synthesis the nutrients are essential so if

more cells are synthesized

then definitely more nutrients present in the wastewater is used for that purpose.

But in anaerobic system since the microbial concentration is so less the nutrients used

for cell synthesis is very very negligible. So, as a haul treatment unit if you take the

anaerobic process there will not be any

nutrient removal like nitrate or ammonia whatever present will be remaining as such but the

one which has escaped through the gas base will be getting removed from the system otherwise

everything will be

getting accumulated in the wastewater treatment or in the anaerobic treatment system itself.

Similarly, phosphorus also will be getting accumulated or it will not be getting removed

from the system. So if you want to use it as a treatment unit and if you want to meet

the effluent discharge standards

whenever we go for an anaerobic treatment system we should have post-treatment. That

is what I have given here; no nutrient removal is taking place in an anaerobic system and

very little pathogens removal is

taking place. So usually after the anaerobic treatment process we give post-treatment and

most of the time the post-treatment is either an activated sludge unit or an oxidation pond

or a maturation pond or an

extended aeration system.

We know that all the minerals or all the inorganic substance whatever is present in the wastewater

will be in the reduced condition when it comes out of the anaerobic reactor. Because in anaerobic

reactor

complete anaerobic condition or an environment which is contusive for reduction exists so

whatever is there it will be coming out in the complete reduced form. So when we discharge

it with the existing water

bodies this reduced form of the minerals or the material will have a tendency to get oxidized

so definitely there will be an oxidation or oxygen demand in the system. So, if you directly

discharge the anaerobic

effluent to a water body there will be some BOD exerted by the treated effluent. So it

is always advisable to go for a post-treatment which can take care of the oxygen demand as

well as the nutrient removal along

with the pathogen removal.

Till now we were discussing about how to remove the organic matter from the wastewater. We

have seen in detail aerobic process, anaerobic process both attached systems and suspended

systems. And when

we were talking about aerobic systems we have seen that in activated sludge process itself

there are various process modification and depending upon the process we are selecting

the sludge and the sludge

generated will be varying.

For example, in a conventional activated sludge process whatever organic matter we are giving

say if you are giving 1 kg of organic matter around 400 to 500 mg will be coming as sludge

or active biomass and

we will be removing in the secondary sedimentation time and it will be coming out as a sludge.

But if you go for extended direction system then most of the biomass whatever is generated

in the system will be

getting auto oxidized and we will be getting only very little amount of sludge.

Similarly in anaerobic process also we will be getting a certain amount of sludge whether

it is microorganism or the inorganic material whatever is present in the wastewater. So

there is a need for the proper

handling of this sludge or sludge treatment is very very essential. Not only in biological

systems but also in physical chemical processes we have seen that whenever we go for coagulation

flocculation what will

happen to the sludge whatever is collected in the flocculation or sedimentation tank

after the coagulation flocculation, so, that sludge also we have to take care.

Now we will discuss about the sludge treatment. What all are the different ways we can handle

the sludge or what all are the precautions we have to take while handling the sludge.

We will talk about the sludge

treatment. Nowadays the sludge treatment is known as

biosolids treatment. Sludge means something not useful or something not desirable. But

once we call it as biosolids that means if you can give some treatment and we

can get something which is having a certain beneficial value then definitely we can call

it as biosolids. In the new or recent edition books they talk about biosolids treatment.

It is in finding out what treatment can

be given in order to get a beneficial use for the end product that we get.

Before going for the sludge treatment we have to see what are the different sources of this

sludge and what is the characteristics of the sludge coming from different treatment

units and what is the amount of the

sludge coming from the treatment unit, so only after finding them we can go for or the

selection of the process which is most economical or most viable for that particular system.

So if you talk about a water

treatment plant or a wastewater treatment plant sludge can be generated from various

sources so these are the solids sources.

First one is screening. In screening we will be getting coarse solids because we know what

the purpose of a screen is and in grit chamber we will be getting grit and in pre-serration

system unit we get grit and

scum again and in primary sedimentation unit we get primary solids and scum.

Primary solids means it will be basically organic matter and in biological treatment

system either it is an activated sludge process or a trickling filter or in anaerobic process

we will be getting suspended solids as

the sludge. In secondary treatment secondary biosolids and scum will be coming and in solid

processing facilities we will be getting solids, compost and ashes. These are the different

sources of sludge or the

biosolids whatever is coming in a wastewater treatment unit.

Now we will see; what are the characteristics of this sludge or the solids we are getting

from different treatment units. The characteristics will be entirely different because each unit

is having its own purpose. For

example, for a screen we know what the purpose of a screen is as it will be removing all

floating and large size particles. And if you compare the grit chamber we know that

its main purpose is to remove grit or

inorganic material which is having high settling velocity.

We know that the purpose of a primary sedimentation tank is to remove the suspended organic matter.

When we come to a biological treatment unit the sludge will be a completely active microorganism.

When

we talk about the chemical treatment units the sludge will be a mixture of pollution

as well as the chemical. So it is essential to know the characteristics of the sludge

before going for a particular treatment. Now

we will see the characteristics of the sludge from each unit.

Screenings: whatever solids collected in the screen will be having all types of organic

and inorganic materials and they will be large in size and the composition varies depending

on the system. This is the

characteristic of the screenings or solids collected in a screen. Now coming to grit

chamber the material collected is what we call as grit, it is heavier inorganic solids

that settle with high velocity because the basic

design itself is meant for removing heavy inorganic materials from the wastewater. If

it goes for further treatment or a secondary treatment unit then it will be adversely affecting

the pumping system because it will

create lot of wear and tear and unnecessarily we will be getting the sludge which is not

active.

So, to prevent that one we provide the grit chamber so this is the basic character of

a grit and sometimes it contains organic matter for example fat and grease. Because if the

settling time whatever we are

providing is very high then definitely along with inorganic material the organic material

also will be settling down so the grit may contain some portion of organic matter like

fat and grease. And this grease can get

attached to the inorganic material and it will be settling or floating along with that

one.

Now coming to scum and grease, we know that in certain cases we put the scum remover or

grease remover. Definitely the solids coming from there is floatable in nature and it consists

of grease, vegetable and

mineral oil, animal fat, waxes, soaps, food wastes, vegetable and fruit skims and its

specific gravity varies from 0.95 to 1. This is the characteristics of scum or grease.

Now we will see the characteristics of primary sludge. whatever we are collecting in the

primary sedimentation tank mostly will be organic in nature and it is easily a digestible

matter because it is not undergoing

any biodegradation there and we know that lot of suspended solids will be present in

the wastewater so in the primary sedimentation tank around 50 to 60% of the suspended solids

present in the wastewater will

be getting removed.

We have seen that the screens and grit chamber will be removing almost all the inorganic

material which is heavier and having high settling velocity. So definitely this primary

sludge will be organic in nature and

easily digestible.

Now we will see the characteristics of the sludge from a chemical precipitation unit.

Usually the sludge will be darkling color and degradable at low rate. The reason is

organic matter will be there and from

inorganic matter will be there so to remove that one we are adding certain chemicals to

change the property of the material or the solids present there so because of that chemical

addition we will be getting

mixture of organic as well as inorganic sludge. Definitely because of the presence of this

inorganic material or the chemical the degradability of the sludge we are getting from a chemical

specification or chemical

treatment plan will be less compared to the sludge we are getting from a primary sedimentation

tank.

Sludge: Usually the color of the sludge will be brown and it is flocculent in nature.

And if the sludge becomes septic then it will become dark. And if the sludge is in good

condition it doesn't have any smell or it is inoffensive and it will be having an earthy

odor. But if it is dark and septic it will

be generating offensive odor. This is the nature of activated sludge and mostly it is

organic matter.

Now coming to the trickling filter we know whatever biomass is present in the trickling

filter. Once it is from the material or the support media what will happen is it will

be coming along with the liquid whatever is

trickling through the bio filter and in the sedimentation tank the biomass will be settling

down so that is the sludge whatever we get from a trickling filter.

The sludge from a trickling filter is usually brownish in color and they are flocculent

in nature. Compared to this activated sludge the trickling filter sludge digests slowly

and sometimes this sludge contains many

worms. The reason is it is getting exposed to flies and as those flies lay eggs on the

trickling filter definitely many worms and larva will be present in this sludge.

If you want to find out the characteristics of aerobically digested biosolids that is

whatever sludge we are getting from these processes either from primary sedimentation

tank, activated sludge process or trickling

filter we can take care of or we can treat the sludge or reduce the volume of the sludge

by further aerobic digestion. So if you go for this aerobic digestion and if you want

to find out what is the characteristics of

the sludge whatever is left over after digestion they will be either dark brown in color, of

course there also flocculent in nature and there is no offensive odor. The well digested

sludge dewaters easily on digestion

beds that is one advantage. If you take the sludge from activated sludge and trickling

filter and if you try to dewater it then it is very very difficult. But once we digestive

sludge then if you want to dewater it then it

will be much easier to dewater it compared to the undigested sludge.

Now we will see the characteristics of anaerobically digested biosolids. Most of the time these

biosolids are completely stabilized and it is black in color and it contains large quantities

of gas and if you want to

find out the property of dried sludge it is having well cracked surfaces resembling that

of garden loam. Hence this anaerobically digested biosolids are the sludge we can directly dispose

off in drying beds

because that doesn't have any odors or anything.

Now coming to compost, compost is a method to treat the biosolids. So, after composing

also we will be getting some solids left over. So the solids whatever we are getting from

composting is dark brown to

black in color and they are well digestive sludge and inoffensive and we can use it as

manure or soil conditioners. This table gives as the basic properties of the sludge whatever

we are getting from various

treatment processors.

If you take the primary sedimentation tank the specific gravity of the solids is in the

range of 1.4 but if you want to find out the specific gravity of the sludge it is only

1.02 because though the solids are having

high specific gravity. We know that the sludge will be containing lot of water around 95

to 96% of water but the solids is only 2 to 4% so the net specific gravity of the sludge

will be very very less almost equal

to the water 1.02.

Activated sludge: Whatever is coming from the secondary sedimentation tank waste the

specific gravity of the solids is around 1.25 and specific gravity of the sludge is only

1.005.

Trickling filter is having a better specific gravity of 1.45 but the sludge specific gravity

is only 1.025.

Extended aeration is 1.3 and it is 1.015 because extended aeration sludge will be mostly inorganic

in nature because whatever organic matter or the biomass present in the system will

be getting auto oxidized

during the aeration process. And in aerated lagoon the specific gravity of the sludge

is 1.3 and specific gravity of the sludge is 1.01 this is the solid specific gravity.

Filtration is 1.2 and 1.005 and if you want to remove the algae from the oxidation pond

the specific gravity of the solid is 1.2 and specific gravity of the sludge is 1.005. These

values are very very important

because it decides the volume of the sludge we have to handle. The volume is very very

important when we design the treatment system. Similarly I have put the specific gravity

of the solids and sludge for

chemical treatment. if you go for line treatment the specific gravity will be 1.9 and 1.04,

if it is high line process it is 2.22, 2.2 is the specific gravity of the solids and

special gravity of the sludge is 1.05 and

de-nitrification 1.22, 1.005 and roughening filters is 1.28 and 1.02.

If you want to find out what is the solids concentration coming out because we have the

sludge and we know what is the volume of sludge coming so if you want to get an idea what

is the amount of solids

present so this cable gives this idea; for primary sludge from a primary settling tank

the percentage of solids varies from 5 to 9 and typical value 6. Similarly I have given

primary sludge with low lime addition for

phosphorus removal the solids concentration vary from 2 to 8 and iron salt addition for

phosphorus removal the solid concentration is 0.05 to 3. So we know what is the treatment

unit and we can get

corresponding solid concentration here.

So we will know what the amount is or what is the mass of solids we have to destruct

and what the volume of sludge is. Similarly this gives the secondary treatment, waste

activated sludge with primary settling

the solid concentration of the sludge varies from 0.05 to 1.5 that means around 98.5 is

water because we will be having such a huge volume and the sludge is only very very little.

The waste activated sludge

without primary settling will be little more because the solids whatever is not removed

in this one because some inorganic solids also will be there so definitely the solids

concentration in the secondary

sedimentation tank will be increasing.

And if you see what is the sludge concentration or solid concentration RBC it is varying from

1 to 3 similarly trickling filter also the range is 1 to 3 and typical design value we

take it as 1.5. Here I have again

given gravity thickener and flotation thickener and centrifuge thickener, the solids concentration

varies from 5 to 10 and 4 to 8. So we can see that in any case we take, the solid concentration

is most of the time

less than 5% and the remaining 95% is water and this is anaerobic digester the primary

sludge if you want to go for the solid concentration we are taking as two to five from waste activated

sludge 1.5 to 4

because this solid concentration is essential especially when we go for the stabilization

unit design.

This table is available in design manuals so we can get a rough idea about what is the

solids concentration usually present in the sludge whatever we are getting from different

treatments units. I have already told

this is essential when we go for the design. We have seen that there will be lot of water

present in that one and there will be different types of solids present in the sludge coming

from various treatment units.

For example, if you take the sludge from an activated sludge process it will be having

both volatile organic matter as well as fixed suspended solids or fixed matter that means

organic and inorganic. So when we

design a stabilization system or a biosolids treatment system we are not going to do anything

with the inorganic solids because we cannot remove that one from the system that volume

will be remaining as such.

But we can remove the volatile solids or the organic solids by biological treatment systems

or by chemical treatment systems. So it is essential to find out what is the volume of

the sludge and what is the mass of

the sludge and what is the mass contributed by the fixed solid and what is the mass contributed

by the volatile solids and vice versa. So we can use this formula if you want to find

out the volume of the sludge or

what is the relationship between volume and mass of sludge.

This formula is nothing but Ws by Ss by row w is equal to Wf divided by Sf into row w

plus Wv by Sv into row w. Here Ws is the weight of total solids and Ss is the specific gravity

of total solids. That means

it includes more volatile as well as fixed solids. And row w as all of know it is nothing

but the density of water and Wf is the weight of fixed solids and Sf is the specific gravity

of fixed solids and Wv is the

weight of volatile solids and Sv is the specific gravity of volatile solids.

So if you know what is the weight of fixed solids and volatile solids and the specific

gravity of fixed solids and specific gravity of volatile solids then we can find out the

volume of total sludge or the weight of

total sludge etc. Now we will see a problem on how to find out the specific gravity of

solids as well as sludge. This is the problem.

One third of the solid matter in a sludge containing 92% water is composed of fixed

mineral solids of specific gravity 2.6 and two thirds is composed of volatile solids,

the specific gravity of solids can be

calculated as given below.

So before that, once again we will read the problem. The sludge containing around 92%

of water and only 8% of solids and in the solids only one third is composed of fixed

mineral solids and two third is

volatile solids. And fixed mineral solid's specific gravity is 2.6 this is the information

we have. Thus, first we have to find out the specific gravity of the solids alone. We are

not considering the water initially but

we are finding out the specific gravity of the solids alone so we can use this formula

1 by Ss is equal to what is the fraction of fixed mineral solids divided by its specific

gravity plus what is the fraction of volatile

solids by its specific gravity.

We know that only one third is fixed mineral solids so this will come as 0.033 divided

by 2.6 plus 0.67 by 1. We are assuming the specific gravity of volatile solids is almost

same as that of water. So we will be

getting 1 by Ss or 1 by specific gravity of the solids as 0.80 or specific gravity of

the solids as such is 1 by 0.80 that is equal to 1.25. This is the specific gravity of solids

alone. But we know that in the total

sludge around 92% is water and only 8% is solids.

Now the next step is we have to find out what is the specific gravity of the sludge as such.

So if the specific gravity of water is taken as 1 then the specific gravity of sludge SsL

this is the solid along with the

liquids or the sludge that we can calculate as follows:

1 by SsL is equal to fraction of solids by its specific gravity plus fraction of water

by its specific gravity. So we know that only 8% is solid so 0.08 by 1.25 + 0.92 by 1 which

is equal to 0.984. This is the 1 by

SsL or 1 by specific gravity of the sludge as such or the specific gravity of the sludge

SsL is equal to 1 by 0.984 which is equal to 1.02.

We can see that the specific gravity of the sludge is almost same as that of water and

only a slight difference is there. And if you want to find out the volume of the sludge

we can use this one. We know what is

the total mass or weight of the sludge divided by row w into SsL into Ps where V is the volume

of the sludge in meter cube, Ms is the mass of dry solids in kilogram, row w is the specific

gravity of water and

SsL is the specific gravity of sludge and Ps is the percent solids expressed as decimal.

So if you know the percentage of solid present as a fraction and mass dry weight then we

can find out the volume of the

sludge.

We have seen how to calculate the specific gravity of the solids as well as the specific

gravity of the sludge. And if you know the solid concentration as weight then we know

how to calculate the volume of the

sludge. All these information are available and we know that from each unit what is the

nature of the sludge that is coming and what is the amount of the sludge coming out and

what is the solid concentration that

is coming out, we have seen all those in detail.

Now we will see the process we have to use for handling the solids. This is the solid

processing flow diagram. It involves lot of processes. I tried to put everything in this

single block diagram. The solid flow is

coming like this and initially it is going for the preliminary operations. The preliminary

operations consist of grinding, degrading blending and storage. we know that the solids

whatever is coming from different

units such as screens, grit etc will be having very high sizes so we have to go for all these

preliminary operations to get uniform characteristic solids.

We know that the water content in these solids will be very very high or the sludge solid

content will be very less. So if you send the sludge just like that to the treatment

unit definitely the treatment unit volume

will be very very high and the chemical requirement will be very high. So it is always advisable

to dewater the sludge as much as possible before sending it to stabilization. So the

second operation in a solid

processing flow diagram is thickening. Thickening is nothing but remove as much as water as

possible. This can be achieved either by co-settling or gravity settling or by flotation or by

centrifuging or gravity belt

process or rotary drum. Therefore all these are the thickening process. We will see each

process in detail afterwards.

Here I am trying to give a basic flow diagram on how we can manage the solids.

Once the thickening is over then we have to go for the stabilization. Stabilization is

the process by which we stabilize the sludge. That means we remove the pathogens present

in the sludge, make the sludge in

such a way that further purification is not possible. So this stabilization can be achieved

either by chemical process that means most of the time we go for line stabilization then

we can use aerobic process and

anaerobic digestion. These are the most commonly used stabilization process. Once the stabilization

is over again the sludge will be containing lot of water so it will be very very difficult

to send it to sludge

drying bed or another application.

The next one is conditioning. Conditioning makes us improve the quality of the sludge

or it will be improving the dewatering property of the sludge, that is the purpose of this

conditioning. Conditioning can be

done either by chemical means or other means either thermal or other processors. Once the

conditioning is over then we go for dewatering. Dewatering as we have discussed here in the

thickening the same

process can be used here either centrifuging, belt filter press or filter press or even

we can go for sludge drying beds or read beds and even lagoons can be used for this dewatering

purpose. The only thing we

have to do is allow the sludge to stay there for long time, with respect to time whatever

water is present in the sludge will be getting out of it and we will be getting relatively

dewatered sludge.

Once the dewatering is over the next step is drying the sludge. Drying can be done either

by direct layer in sludge drying beds or indirect layer method or composting alkaline stabilization

and pasteurization long

term storage. All these processes can be adopted for the heat drying process. After that one

if you want to further reduce the sludge volume we can go for thermal reduction. That means

we can go for different

types of incineration process because we have got the sludge already thickened and dewatered

and a dried one. We can apply to the land and if you don't want to use that process

and if you want to further

reduce the volume we have to go for thermal treatment or incineration. Various types of

incinerators can be use for this purpose.

Now we will see the processes we usually use in wastewater treatment plants. If the waste

activated sludge is coming from an activated sludge process first we go for a thickener.

We have seen in detail how to

design a thickener. The secondary sedimentation tank itself will be acting as a thickener.

So here the sludge from the activated sludge process as well as the sludge from the primary

sedimentation tank can come

and the thickening can take place then it will be going to anaerobic sludge digester

and whatever supernatant is coming will be separated and sent for further treatment and

whatever is the sludge coming from the

anaerobic treatment system or the anaerobic digester usually it undergoes chemical conditioning

if you are going for belt filter press for the dewatering.

This is because chemical conditioning is required. When we discuss in detail we will know why

this chemical conditioning is required. Once the dewatering is completed it can go for

disposal or for further

processing or land application. So this is one of the methods we can adopt for handling

the sludge from a waste activated or the sludge from an activated sludge process. This is

another system. This is a

combined thickened waste activated and primary sludge so it is going for anaerobic sludge

digestion then chemical conditioning. Here the only difference is instead of going for

a belt filter press we can go for a

centrifuge so definitely the centrifuge also will be dewatering the sludge and we will

be getting dried sludge and we can send the dried sludge for disposal or for further processing

or for land application.

This is the third alternative the combined waste from the activated and primary sedimentation

tank is coming and again we are going for anaerobic digestion and the plant effluent

whatever is coming is going for

further treatment.

Here we are not going for any chemical conditioning or anything because the drying step is not

either centrifuging or belt filter press, we are using only drying beds. So we are allowing

the sludge to retain or stay

there for a long time so within that time whatever water is present in the sludge will

be drying out and because of the solar radiation or the temperature coming from the solar radiation

the remaining water will be

getting evaporated and we will be getting a dried sludge which can be disposed off or

it can be process further or we can use it for land filling.

The flow chart we have seen so far tells us what are all the steps involved and we have

seen them in detail. We have seen the process required for complete handling or complete

treatment of these biosolids or

the sludge generated from the various treatment operations and processes of wastewater. Now

we will see each process in detail. First we will talk about the preliminary operations.

As we have discussed earlier the preliminary operations consist of grinding, degritting

etc. What is grinding, degritting, blending and storage of solids?

These are the preliminary treatments. As we have already discussed the purpose of this

preliminary treatment is to get a relatively homogeneous feed to the subsequent facilities.

This is the major objective of

these preliminary operations. What we are doing in grinding or why grinding is essential?

We know that the solids whatever is collected in the screens will be large in size so grinding is essential.

Large and stringy materials are cut or sheared into small particles to prevent clogging or

wrapping around

rotating equipment. If you want do this one what will happen is that the large particles

will be going to the next or the other treatment units so it will be clogging the perms and

other operating systems which is

having moving parts so it is essential cut them or shear them into smallest pieces.

Therefore, this grinding is essential for the screenings because the screening will

be removing the nuisance material. Even after grinding some floating material which is larger

in size which is not grinded properly

will be getting removed in the screens and in degritting what we are doing is if you

don't have a grit chamber or if the grit chamber is not functioning properly then definitely

this inorganic material will be coming

along with the biosolids so the sludge will be containing both organic and inorganic solids.

So if the grit or the inorganic solids are present along with the sludge then the pumps

whatever we are using for the sludge will be getting affected by this grit so it is

very very essential to remove the grit or the

heavy inorganic material whatever is present in the biosolids. So it is necessary when

no grit chambers are provided before the primary settling. The purpose is to remove the grit

whatever is present in the

biosolids.

Blending is the process in which the mixing takes place so that finally we get a uniform

characteristics sludge. We are grinding it then we are screening to remove the nuisance

material then we are going for

degritting. Degritting will be removing all the grit material. Once these three operations

are done we will be blending the sludge because we have seen the characteristics of the sludge

coming from different

operations are entirely different. So if you send them one by one so the treatment system

the treatment system will be getting the feel of so much varying characteristics so it will

be affecting the efficiency of the

treatment system further whatever we are going to adopt for stabilization and other processes.

So it is essential to send some what homogeneous sludge to further treatment units so that

is the purpose of this

blending.

Storage: to smooth out fluctuations in the rate of solids production. we know that the

solids whatever is coming from different treatment units will not be continuous in nature because

the screens or the screening

whatever we are getting and whenever the screens get clogged and when the pressure drop is

more than the permissible limit then only we go for cleaning of the screen. Once the

screen is clean we are getting the

solids from the screen so definitely it is not a continuous one.

Similarly it is for the activated sludge process also. in most of the cases the sludge wastage

is not on continuous basis, it is daily once or twice or three or four times we will be

wasting the sludge. Similarly in

other biological process and chemical process the sludge wastage will not be a continuous

one. So in such cases if we allow the sludge whatever is coming from the treatment units

as such to the biosolids

handling units then the efficiency of the system will not be proper. In such cases what

we have to do is we have to blend the sludge and store it in a storage tank which will

be utilizing the flow of the sludge and

from the storage tank we can supply a specified quantity or a uniform quantity to further

treatment units. So the sludge storage tank will be functioning exactly in the same way

as that of an equalization tank in a

wastewater treatment system. This storage is also very very essential which smoothes

the fluctuations in the rate of solids production.

Now we will talk about thickening. The preliminary process is over and we know that the solids

whatever is coming will be having less than 5% of solids. So if the solids concentration

is less than 5% we can

imagine that the remaining 95% is water because of this water content the volume of the treatment

unit whatever we are going to provide will be increasing considerably. Definitely the

treatment cost or the capital

cost for the treatment unit will be increasing drastically. So if you can reduce the water

content considerably then definitely your further treatment unit volume will be reducing

significantly, that is the very purpose

of this thickening.

The main function is volume reduction by sludge concentration and the advantage of this thickening

is the capacity of tanks and equipment required can be reduced because we know that the solids

is only less

than 5% and the remaining capacity is only to handle the water whatever is present in

the solids. So if you can remove that one we are only interested in handling the solids

so the capacity of the tanks and

equipment required can be reduced considerably then definitely the quantity of chemicals

required can be reduced.

The reason is if you go for chemical conditioning or chemical stabilization if you want to add

the chemicals definitely we have to add the chemicals to take care of the water whatever

is present in the sludge

because if the water content is small the chemical whatever we are adding it will be

getting diluted and the effect will be less. Hence, we add more amount of chemical to take

care of the dilution aspect. So if you

can remove the water content from the sludge considerably definitely the quantity of the

chemical to be added can be reduced significantly.

If you talk about the heat treatment the amount of heat required can be reduced because if

the moisture content or the water content is very high definitely the heat required

will be very high. So if you can remove

that one the energy involved or the heat that we have to supply to achieve the same degree

of treatment can be reduced considerably.

What are the methods commonly used for thickening? They are;

Co - settling thickeningGravity thickening

Floatation thickeningCentrifugal thickening

Gravity belt thickening andRotary term thickening

These are the most commonly used methods for thickening.

In co-settling we will be having various clarifiers and the sludge from the clarifiers will be

coming to the thickener. Similarly the overflow from the clarifiers also will be coming to

the thickener. So because of this

one a proper thickening will be taking place in the thickening tank and it will be considerably

reducing the sludge volume.

Gravity thickening we have already discussed in detail when we were discussing about the

secondary sedimentation tank design. In activated sludge process whenever we design the secondary

sedimentation tank

it has to satisfy two criteria, one is the clarification and another one is sludge thickening.

So we have to find out the area of the secondary sedimentation tank based upon these two criteria

and we have to provide

the area whichever is coming, mark either area corresponding to the clarification or

the thickening.

The thickening area we can find out using the solid flux theory and we have seen in

detail how to find out the area and how to design a gravity thickening unit. This shows

the picture of a gravity settler. It is

nothing but a sedimentation tank but the only thing is when we design the overflow rate,

loading rate and cross sectional area it should be provided based upon the sludge thickening

characteristics.

This is a circular type sedimentation tank, this is the entrance of the sludge it will

be getting distributed like this, this is the pressurized wastewater entrance it is

coming here and here it is getting distributed like

this and here it is getting settle and clear liquid will be going out like this and sludge

can be collected through this one this is the sludge collector and here we have a retention

baffle this will help the solids to enter

into the liquids so what will happen is more and more clear liquids will be getting remote

from this system so definitely whatever solids we are getting here will be having a high

density compared to whatever we

are supplying.

So this is a thickener floatation unit we all know. if the solid's density is less then

instead of going for gravity settling or settling we can concentrate it by floatation. So floatation

also we have discussed in detail

when we were discussing about the solid removal. Centrifugal thickening: Here we use the centrifugal

force for the thickening. And in gravity belt thickening the sludge will be placed on a

belt and the belt will be moving and with respect to time what will happen is

whatever water is present in the sludge will be gradually oozing out and we will be getting

a relatively concentrated sludge towards the end of the belt thickener.

In rotary drum thickening the drum will be rotating and because of the rotation the liquid

will be getting remote from the sludge and we will be getting a concentrated sludge.

I will show few photographs of this centrifugal thickening gravity belt and rotary drum thickening.

This is the centrifugal thickener.

Here we can see that the free solids are entering and it is getting out in the system here,

it is coming through the central shaft and it is getting out here and as we all know

this is the gear box and this gear box will

be rotating at a differential speed so with respect to time the solids will be getting

pushed to the periphery of this drum, this is the rotating drum and we can see that this

is having a tapering section. So as the

solids move towards this side more and more compression will be taking place and the water

will be getting discharged from the sludge and here we will be getting somewhat concentrated

sludge. This we can

remove using this part here and here so the drum will be rotating and the feed is coming

from the center and because of this one it is getting thickened.

So this is the centrifugal thickener and this is the belt thickener. Here we can see that

the liquid sludge is coming, we are adding some polymers and that is coming here, it

is moving through the belt and because

of this one gravity drainage is taking place so water is getting removed and this portion

is meant for belt washing . So as it moves the sludge will be getting dewatered and here

we get more concentrated sludge.

We have seen in detail the different process we have to adopt for the solid stabilization.

we have seen the flow diagram so preliminary treatment is required then we have to go for

dewatering the reason is the

water content in the sludge will be very very high and solid concentration will be less

so definitely it will be increasing the volume of further treatment. So once the dewatering

is done then we can go for

stabilization. The purpose of stabilization is to remove the pathogens and to distract

the organic matter so that there will not be any further futrification.

Once the stabilization is over we can go for conditioning and again dewatering then drying

then disposal. but in usual wastewater treatment plants especially when we go for the biological

treatment systems we

have seen that either in the activated sludge or the sludge coming from trickling filter

usually we go for dewatering then anaerobic digestion then if you are going for belt thickener

or centrifuge we have to go for

chemical conditioning.0

But if you are going for sludge drying beds this conditioning is not required because

in the sludge drying bed itself we are giving lot of time for the water to get removed from

the sludge so that we can get a dry

sludge which poses less volume and whatever is coming out by this we can use it as a manure

because it is almost degraded and it will be having high nutrient value or if you want

to further reduce the volume of

the sludge we can go for heat treatment like incineration etc.

Once the volume reduction is taken place we can dispose it into land like and filling

or something like that. So in the next class what we will do is we will be discussing in

detail what all are the stabilization process

that are most commonly used because that is very very important. We have to design the

stabilization process based upon the sludge nature and the sludge quantity. So we will

see the sludge stabilization and

other processes in detail.

The Description of Lecture 29 UASB (Contd) Sludge Treatment