Encyclopedia of Environmental Science and Engineering, Volume I and II

BIOLOGICAL TREATMENT OF WASTEWATER 145

as the ratio between total active microbial mass

in treatment system, X T , and total quantity of

active microbial mass withdrawn daily, includ-

ing solids wasted purposely as well as those

lost in the effluent, ∆ X T / ∆ t. Regardless of the

fraction of active mass, in a well-mixed system

the proportion of active mass wasted is equal

to the proportion of total sludge wasted, mak-

ing sludge age equal for both total mass and

active mass.

U – Process Loading Factor, or Substrate Removal

Velocity, or Food to Microorganisms Ratio,

or Specific Utilization, is defined as the ratio

between the mass of substrate utilized over a

period of one day, ∆ S / ∆ t, and the mass of active

microorganisms in the reactor, X T.

t ̄ – Hydraulic Retention Time or Detention Time,

or Mean Holding Time, is defined as the ratio

between the volume of Reactor, V, and the volu-

metric feed rate, Q.

B V – Volumetric Loading Rate or Hydraulic Loading

Rate is defined as the ratio between the mass of

substrate applied over a period of one day, S T / ∆ t

and the volume of the reactor, V.

E – Process Treatment Efficiency or Process Perform-

ance is defined as percentage ratio between the

substrate removed, ( S 0 – S e ), and influent sub-

strate concentration, S 0.

A desired treatment efficiency can be obtained

by control of one or more of these parameters

separately or in combination.

5. BIOLOGICAL TREATMENT SYSTEMS

The existing biological treatment systems can be divided

into the following three groups:

(a) Aerobic Stationary-Contact or Fixed-Film Sys-

tems: Irrigation beds, irrigation sand filters,

rotating biological contactors, fluidized bed

reactors, and trickling filters fall in this group. In

these treatment processes, the biomass remains

stationary in contact with the solid supporting-

media like sand, rocks or plastic and the waste-

water flows around it.

(b) Aerobic Suspended-Contact Systems: Activated

sludge process and its various modifications,

aerobic lagoons and aerobic digestion of sludges

are included in this group. In these treatment pro-

cesses, both the biomass and the substrate are in

suspension or in motion.

(c) Anaerobic Stationary-Contact and Suspended

Contact Systems: Anaerobic digestion of sludges

and anaerobic decomposition of wastewater in

anaerobic lagoons fall in this category.

A typical layout of a wastewater treatment plant

incorporating biological treatment is shown in Figure 12.

Primary sedimentation separates settleable solids and the

aerobic biological treatment is designed to remove the sol-

uble BOD. The solids collected in primary sedimentation

tanks and the excess sludge produced in secondary treat-

ment are mixed together and may be digested anaerobically

in digesters. Trickling filter and activated sludge processes

are most common secondary treatment processes for aer-

obic treatment and are discussed in detail. Discussion of

sludge digestion by anaerobic process and use of biologi-

cal nutrient removal as a tertiary treatment have also been

included.

In addition to conventional pollutants present in munici-

pal and industrial wastewaters, significant concentrations of

toxic substances such as synthetic organics, metals, acids,

bases, etc., may be present due to direct discharges into the

sewers, accidental spills, infiltration and formation during

chlorination of wastewaters. It is import to have a knowl-

edge of both the scope of applying biological treatment and

the relevant engineering systems required to achieve this

capability. Thus, the kinetic description of the process and

the deriving reactor engineering equations and strategies for

treatment of conventional and toxic pollutants are essential

for proper design and operation of biological waste treat-

ment systems.^24

I- Completely Mixed-No biological solids recycle

II- Completely Mixed-Biological solids recycle

III- Plug Flow-Biological solids recycle

Q,So

Q,So

Q,So

Q,X,Se

X, Se

X, Se X, Se

X, Se

Reactor

Reactor

Reactor

(Q+Qr)

(Q+Qr)

(Q–W), Se

(Q–W), Se

Xe

Qr, Xr, Se

Qr, Xr, Se

w,Xr

w,Xr

Settling

Tank

Settling

Tank

Sludge

Sludge

FIGURE 11 Treatment models.

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