Programmable Logic Controller
Automated Sewage Treatment Plant
1. TECHNICAL FIELD OF THE INVENTION
[OOI] The present invention relates to commissioning, re-commissioning,
maintenance and operation of a sewage treatment plant (STP); in particular,
it describes a novel method of automated protocols for these operations
executed through programmable logic controller (PLC).
2. BACKGROUND AND THE PRIOR ART
[002] It is unimaginable to sustain the present day urban-based civilization without
a well-functioning sewage treatment plant around major population centers.
Since early 1900s, the practice of treating sewage started replacing the
erstwhile open defecation system when it was demonstrated that sewage
borne bacteria were responsible for many infectious diseases. However,
primitive sewage treatment methods were limited to acquiring large tracts of
land to spread human excreta and other biodegradable wastes to let them
decay under the action of micro-organisms.
[003] As cities became larger, pressure on land and water conservation increased
leading to development of sewage treatment equipment that required
comparatively smaller space.The sewage treatment plants today come in
variety of sizes and complex configurations depending on the size of
population they serve.
C
[004] The main purpose of all sewage treatment plants is to take all used water
I and waste from municipal sewage lines and run it through a cleaning
process to remove solid particles and other pollutants, chemicals, harmful
bacteria etc.beforereleasing it back into the environment. Irrespective of
size and operative capacity, the general layout of a sewage treatment plant I
can be shown as in Figure 1; the objectives of various treatment steps
involved can be briefly described as follows:'
[005] The objective of the preliminary and primary treatment step is to remove
suspended and floatable material from incoming wastewater at the
treatment facility by passing it through large bar screens and allowing it to
flow in to settling tanks. Here, the solids settle down at the bottom where
sludge is formed and collected; some oily materials, if present float to the
surface which is usually removed by skimming. The secondary sewage
treatment involves treatment of biodegradable organics present in the waste
waters to reduce the 'biochemical oxygen demand (BOD)' to prescribed
safe limits for fish and other aquatic life in receiving water bodies where the
treated effluents finally reach.
[006] Biological treatment of waste waters takes place in fixed media or
suspended growth reactors using activated sludge. More commonly, the
effluent from primary wastewater treatment is mixed with activated sludge in
aeration tanks. Activated sludge has microorganisms to decompose
organics. A tertiary sewage treatment is sometimes carried out to eliminate
disease-causing pathogenic micro-organisms. As a final step, chlorine is
added to the water to kill any remaining bacteria before discharging.
[007] The design and layout of a sewage treatment plant is decided on the basis
of the extent of treatmentrequired which is determined by comparing the
influentwastewater characteristics and the required effluent wastewater
characteristics as per applicableregulations. The contaminants in
wastewater are removed by a combination of physical unit operations,
followed by chemical and biological unit processes.The physicalunit
operations may include screening, mixing, flocculation, sedimentation,
floatation, filtration, gas transfer etc. And the chemical and biochemical unit
processes may include precipitation, adsorption, bio-conversion etc.There
could also be series of bio-conversion and fermentation processes
depending on the type of toxicants to be dealt with besides dealing with
household carbon source. Finally, important engineering factors such as
topography of the area, its slope and terrain, available hydraulic head,
groundwater depth and soil characteristics all come under consideration.
[008] While, in principle, the sewage treatment plants conform to a general and
simple lay out, in practice, however, they are getting more and more
complex by the day due to expanding knowledge base on all aspects of
sewage treatment. A recent review'of emerging technologies in this sector
m merging Technologiesfor Wastewater Treatment andln-Plant Wet Weather Management, US EPA,
March 2013
provides a status of up-coming technologies; some of these technologies
are grit removal screening, solid removal, BioMEMS, handheidadvanced
nucleic acid analyser, immuno-sensors and immuno-assays, ultra-fine
bubble diffusers, solar drying of sewage sl~dge~filtrationin cluding
microfiltration and ultra-filtration through membranes,andvariety of
anaerobic processes,viz., up-flow packed-bed attached growth reactor, upflow
attached growth anaerobic reactor, anaerobic expanded-bed reactor,
down-flow attached growth process, anaerobic contact process, anaerobic
sequencing batch reactor, up-flow anaerobic sludge blanket process,
anaerobic fluidized bed reactor etc.
[009] Despite growing complexity and advancements on technological fronts, the
ground reality with regard to control and management of sewage treatment
plants, especially in many cities of developing countries presents a grim
picture. While the installed capacity is much less than the total waste
generation in many places, few among those plants that have been installed
either run over loaded or under loaded.Most sewage treatment plants tend
to have minimal automation and elementary control systems for flow
metering and depend on the experience of plant operators. Further, the
skilled and experienced plant operators are generally in short supply.
[OOIO] Management, operation and control of sewage treatment plants are of great
concern which havenotreceived adequate attention so far. Mostly problems
in the operation of sewage treatment plants are logged manually and
addressed by plant operators intuitively. Occasionally, alerts are sent from
the site to the vendors of OEMs to come and rectify the problems being
encountered. Often, such situations also result in temporary shutdowns of
the plants. Significant variations in the composition of the wastes and
wastewater arising from different clusters of domestic population and
industries may also create difficulties in ensuring the efficiency and
effectiveness of required BOD and TSS removal. Other factors that
contribute in affecting the efficiency are temperature, flow of wastewater, pH
and presence of different components.
[ O O I I ] Parameters like air pressure and water pressure are seldom monitored in
the pipeline within the sewage treatment plants of the prior art. Further,
commissioning a sewage treatment plant continues to be largely a manual
process that involves starting and stabilizing a bioreactor by building up of
bacterial mass in the reactor with intermittent addition of carbon source and
continuous aeration. Nonetheless, commissioning is a rigorous process
requiring continuous monitoring and dedicated attention of qualified and
skilled manpower to follow a rigid protocol. In most cases, however, the
startup process is left to the plant operator's discretion which is seldom
successful and complete. Given the skills and motivation of plant operators
as commonly prevalent, the commissioning process is invariably incomplete
with regard to sewage treatment and the reactor is left to stabilize on its own
stretching over a period of several months. Further, reaction kinetics of
biochemical conversionscan only be optimized by frequent pH adjustments
which are also not possible without required instrumentation and skilled
manpower.
[0012] Incidentally, commissioning of a sewage treatment plant and starting the
bioreactor is not just one time activity. The sewage collection system and
weather conditions could significantly influence the overall quantity, speed
and composition of influent coming into the plant and there could be
frequent reactor upsets. Such episodes lead to temporary shut-downs and
re-commissioning of the plant which tends to be as problematic as the
original commissioning process and is quite manpower intensive in the prior
art. A reliable and automated control system for commissioning and
management of sewage treatment plant is therefore quite essential which
has been addressed and disclosed in this invention.
[0013] Recent patent literature describes some attempts by various inventors to
develop and integrate in-situ or remote monitoring systems with sewage
treatment plants through various approaches. For example, US7626508
and US7768413 describes a monitoring device as an integrated unit that
includes sensors, a two-way telemetry unit, a power supply, a processor,
and supporting hardware, all located in an enclosed, waterproof
housingplaced within a manhole cavity to obtain information about water
level communicated to a remote monitoring station. This invention aims at
detecting water level crossing safe limits.
3 [0014] Another invention by a Korean inventor patented in USA (US7862722) and
in India (lN253623) describes a sewage treatment system using a control
device comprising of measuring units having sensors to detect and measure
specific components of the sewage. The signals thus obtained are
validated and controlled at target values through a proportional-integralderivative
(PID) control unit for process optimization.
[0015] The monitoring and control system of a sewage treatment plant, as
described inJP2003-201097 uses inlet water quality sensor as well as outlet
I water quality sensor along with inlet and outlet flow meters and control
valves; the measured data being communicated to a central monitor/control
centre. Another monitoring control system for sewage treatment plant as
reported in JP2008-152377 also measures greenhouse gas (COz)
discharge in addition to treatment and measurement of quality of water.
[0016] The biological processes in a sewage treatment plant as described in
DE4140915 is controlled for regulating the rate of addition of oxygen
through fuzzy logic based on inexact input data measurement, e.g., degree
of foaming in a treatment tank as judged by an operator as light, heavy or
moderate. Another monitoring and control process as described in
DE10034645 for a biological sewage plant with a highly variable
composition of sewage from different sources uses neural network
computer to generate control signals for mixing sewage of varying
compositions after determining their properties and that of activated sludge,
especially its oxygen consumption capacity. A system and method for
controlling treatment of the sewage 1 waste water as reported in US6808630
1 EP1376276 uses a neural network control program with a backpropagation
algorithm on measured data of inflowing water into the sewage
treatment plant and various attributes of reaction tanks including dissolved
oxygen (DO), solids retention time (SRT) etc.
[0017] A remote monitoring system of a sewage treatment facility as designed and
described in JP 09-120412 provides automation of operations through a
host computer located in a central control centre connected through
telephone line with a slave computer on site fed by data collected by
monitoring equipment at the treatment facility and relay pump station.
Process is controlled by manipulating control valves performed by remotely
changing the parameters from the host computer through dedicated
treatment function diagnosis software.
[0018] There are some patents in the prior art reporting the use of programmable
logic controller to monitor and control certain parameters of sewage
treatment. US5321601, for example, incorporates a PLC along with level
measuring device and hydraulic system to control a gate valve. A data
display screen also allows inputting set point changes. KR100809382 also
employs a PLC board to control various devices of sewage and wastewater
treatment system comprising of a flow rate-regulating bath, a contact
oxidation bath, and a membrane separation bath.
[0019] EP2106834 describes a PLC controlled sludge dewatering and wastewater
filtration. CN201867642, a utility model patent describes a fully automatic
sewage treatment and monitoring system comprising of a main machine for
a central monitoring room, and a primary, secondary and tertiary PLC
monitoring devices all connected with one another through a high-speed
communication network. US7335305 describes the use of PLCs for
controlling the aerobic phase of a wastewater treatment cycle with at least
two cycles for a sequencing batch reactor.
[0020] The present invention describes the use of PLC for commissioning and
trouble shooting of a typical SBR based sewage treatment plant. Further, a
comprehensive process control as described in this inventionis also
absolutely novel. Algorithms of logic for several key processes of the plant
constitute its inventive features reported for the first time in this invention.
3. OBJECT OF INVENTION
[0021] The primary object of this invention is to design a PLC programmed system
for commissioning and re-commissioning of a sewage treatment plant
based on sequential batch reactor.
[0022] It is yet another object of this invention to design a PLC programmed
system that can automatically find and repair a fault in a SBR based
sewage treatment plant.
h
I 4. SUMMARY OF THE INVENTION
[0023] The A sewage treatment plant based on sequential batch reactor has been
automated with programmable logic controllers (PLC) for process control.
The plant can be commissioned by providing four user-defined variables
initially, viz.,) sewage filling time, ii) aeration time, iii) settling time and iv)
decantation time. In its preferred embodiment, the plant is designed for
sewage filling time of 90 min.
(00241 However, during the initial start-up commissioning of the plant, sewage
filling time at first is set at far in excess of 90 min. and gradually reduced to
90 min through successive batch until the process stabilizes. After a few
successive batches in 'commissioning mode', the plant can be switched to
run in 'auto mode'. PLC of the plant is also programmed for being able to
detect faults such as 'decant failure', 'decant incomplete', 'blower & raw
sewage pump failure' etc.
5. STATEMENT OF INVENTION
LO0251 The operation of a sewage treatment plant based on sequential batch
reactor, including commissioning, re-commissioning, fault detection and
rectificationof process failure as well as plant maintenance which has
hitherto been largely a manual process controlled by operating personnel is
now automated through this invention using programmable logic controller
to control key processes and related hardware components.
- 11
I 6. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING(S)
I [0026] Having thus described the invention in general terms, reference will now be
made to the accompanying drawings, which are not necessarily drawn to
scale, and wherein:
Figure 1 shows a general layout of a sewage treatment plant as part of the
background to this invention
Figure 2 shows various tanks and components in SBR system
Figure3 shows the sequence & steps involved in a Sequential Batch
Reactor based STP
Figure 4 shows the process flow diagram of SBR based STP for
development of operational logic
Figure 5 shows the commissioning mode PLC logic algorithm
Figure 6 shows the auto mode PLC logic algorithm
Figure 7 shows the air blower logic algorithm
Figure 8 shows the filter feed pump operation logic algorithm
7. DETAILED DESCRIPTION OF THE INVENTION
[0027] To overcome the problems related to operation and maintenance of sewage
treatment plants as described in the background and to meet the
shortcomings of the prior art in this field, this invention provides automation
of the processes of the sewage plant.
[0028] To meet the objectives of this invention from the technological standpoint,
following pre-requisites were considered to be met.
i. The expert system of the process control must govern all elements of
the sewage treatment plant performance, and monitor its proper
functioning.
ii. The program of the expert system must be able to handle daily
operations as well as emergencies and maintenance issues.
iii. The program should provide clear and detailed system status, based
on records and alarms, to facilitate decisions making through human
intervention, when necessary.
[0029] In the preferred embodiment of this invention, programmable logic
controllers (PLCs) have been employed for real-time control of key
processes in the sewage treatment plant based on sequential batch reactor.
Theoperational scheme of this embodiment of a sequential batch reactor
with required hardware architecture as well as symbolic control levels are
shown in Figure 2. The knowledge based expert software system
embedded in the PLCs is fed with data obtained from the in-line plant
monitoring as well as pre-determined control levels (set-points).A person of
ordinary skilled in the art will appreciate that many variations and alterations
to the embodiments described herein are within the scope of the invention.
Accordingly, the following embodiment of the invention is set forth without
any loss of generality to, and without imposing limitations upon, the claimed
invention.The use of PLC for commissioning and trouble shooting of a
typical SBR based sewage treatment plant and a comprehensive process
control as described in this invention is, however, absolutely novel and has
several inventive features reported for the first time in this invention.
[0030] Programmable Logic Controller: Real time control for operation of sewage
treatment processes is, now a days, becoming as important as in other
sophisticated processes, i.e., chemical processes, equipment interlocks,
smoke detection, gas monitoring, machine protection, personnel safety etc.
where continuously monitored process variables are used to control the
process through control loops, controllers and actuators.
[0031] A sewage treatment process can be controlled in real time if process
variables are monitored in the systemand continuously used to operate
actuators during the process. These processes, in terms of quantity, may be
flow rate, storage, pumping etc. and, in terms of quality, may be
sedimentation, sludge; treatment etc. The control of these processes can be
schematized by means of control loops and implemented by means of
hardware components sensors, actuators, controllers and data transmission
systems. While sensors monitor the process evolution and actuators
influence the process, the real intelligence of process control is provided by
the controller as programmed to achieve minimum deviations of the
controlled process variable from its desired value (set-point).
[0032] Among different type of controllers, digital programmable logic controller
(PLC) which is slowly replacing the use of analogue controllers, controls all
functions of acquisition of measurement data, pre-processing, verification of
status and limits, data storage and calculation of control action. It can also
receive and report data from and to the central station. It also provides
computer screen for display of standard features showing real-time values,
trends and alarms which are far more user-friendly than erstwhile wallpanels.
PLC is also much less complex than other relay based systems and
being modular type, it is also very flexible due to network architecture and is
also very space and cost efficient.
[0033] Sequential Batch Reactor: Having described the sewage treatment plant in
general terms in the background, a brief description of Sequential Batch
Reactor (SBR) (Figure 2) is in order since an embodiment of this invention
applies to this type of sewage treatment plants. SBR is an aerobic
biological process to oxidize organics with the help of heterotrophic
bacteria. The SBR process utilizes a fill and draw reactor with complete
mixing during the batch reaction step (after filling) and where the
subsequent steps of aeration and clarification occur in the same tank. The
SBR process has five steps which are carried out in sequence as follows
and graphically shown in Figure 3:
i. Fill
ii. React or Aerate
iii. Settle or Clarification
iv. Draw or Decant
v. Idle.
[0034] The tanks and various components for a SBR system are shown in Figure
2. Raw Sewage Tank serves to equalize the variations in the hydraulic flow
and the variations in the organic load. These are generally designed to hold
six hours of average flow. Sludge Tank stores excess sludge that is
generated in the SBR process before it is fed into the Filter Press for
dewatering. Generally, sludge holding tank is designed to hold excess
sludge that is generated in about three days. Sequential Batch Reactor is
the aeration tank in which the oxidation of organics to carbon dioxide
occurs. The aeration tank is generally designed with a hydraulic retention
time of twelve hours and the number of batches may vary from two to eight
batches a day. The treated sewage from the reactor will then be transferred
to the filter feed tank by gravity during the draw or decant phase of the
process. A filter feed pump then transfers the treated sewage from the filter
feed tank into the sand and activated carbon filters that remove the
suspended solids and fine organics respectively. The treated sewage is
then stored in the treated water tank before the water is reused in gardening
and other horticulture applications.
[0035] Best Mode of Enablement of the Invention - Loqic Development : The best
mode of enablement of this invention lies in development of logic for
commissioning of the SBR based sewage treatment startup process and
operation and maintenance of the plant in auto mode with the option to recommission
and fault detection and repair when necessary. Such a plant is
then erected which duly equipped with PLCs programmed as per the
developed 'logic'as well as required instrumentation. A process flow sheet
for such a sewage plant for development of required logic is shown in
Figure 4 and the logic algorithm developed for various functions is
described hereunder.
[0036] This embodiment of sewage treatment plant has five processes that operate
sequentially or selectively as explained earlier. The necessity to activate
some or all of these processes depending on the need which hitherto
required the skill and judgment of the operator has now been programmed
in this invention to be managed by the PLCs.
[0037] Commissioninrr Mode Loaic:This mode is engaged by the user when the
STP is to be commissioned. This mode essentially controls the four
processes, i) sewage filling, ii) aeration, iii) settling and iv) decantation by
regulating time taken by each process. In this embodiment, the plant is
designed for sewage aeration time of 90 min. However, during the initial
start-up commissioning of the plant, aeration time at first is set at far in
excess of 90 min. and gradually reduced to 90 min through successive
batch until the process stabilizes. The logic algorithm for 'commissioning
mode' is show in Figure 5.
[0038] As per the algorithm shown in Figure 5, when commissioning mode is
activated, the PLC reads the availability of sewage in the equalization tank
through RSTI. When the sewage is not available for commissioning at any
time during the commissioning mode, the PLC flashes "No Sewage" and
halts commissioning till sewage is available to reinitiate commissioning
process by level sensor RST2.
I . [0039] When the sewage becomes available as per RSTI, the commissioning
mode becomes operable with following four key user defined variables:
R = time to fill raw sewage; Q1 = aeration time; D = settling time and
F = decanting time. As soon as the sewage is available during the
commissioning process, the PLC initiates aeration for user definedUQn" min
(90 <= Qn<=999). After Qn elapses, raw sewage is filled into the SBR tank
from the equalization tank for user defined Fill time (R = Usually Time taken
to fill 118'~ of the SBR tank volume). After time "R" elapses, the PLC checks
the status of the Top level sensor in the SBR tank (SBR3). If the status is
open meaning that the SBR tank is not full, then aeration is in for "Qn" min
followed by filling for R min. This cycle of aeration and filling continues till
the SBR tank is full.
[0040] If the SBR tank is full which is read from the status of the SBR3, the PLC
starts settling phase for a defined time of "Dl' min. After D elapses the
treated water is decanted. This decantation phase completes one complete
commissioning batch. Before the next commissioning batch is initiated, the
commissioning aeration time is reduced by 100 Min (ie Q1 = Qn-100) and
the filling time is unaltered. If the new aeration time Q1 > 90 min , then the
next commissioning batch is carried out with the new aeration time Q1 and
Filling time "R". The aeration and filling phases continue till the SBR tank is
full which is acknowledged by the level sensor "SBR3". These are then
followed by the settling and decant phase and the end of decantation
completes the next commissioning batch. The aeration time is again
reduced by 100 min (ie Q2 =Q1-100) which will be the new aeration time for
the next commissioning batch. The batches continue with every
commissioning batch having aeration time reduced by 100 min than of the
previous batch. This reduction in the aeration time happens till the aeration
time is greater than 90 min. The aeration time does not get reduced below
90 min as minimum aeration time if fixed at 90 min.
[0041] The PLC then carries out the commissioning batches with an aeration time
of 90 min and filling time of "R min" till the user commands to get out of
commissioning mode manually. When the user concludes the
commissioning procedure, the SBR can now be put into the auto mode by
the user.
[0042] Auto Mode Lonic: Auto mode enables the operation of the sequential batch
process by a set of commands in a sequential manner with adequate and
essential check points to confirm the safety and completion of treatment
process. Having described the process steps of a typical 'Sequential Batch
Reactor' earlier in this description and graphically shown in Figure 3, the
PLC executes the process in steps with multiple safety check points as
explained in the Logic diagram in Figure 6. The Process starts with the
feedback from the level sensor RSTI in the raw sewage collection tank. If
the availability of the sewage is confirmed by the RSTI the PLC seeks
signal from the RST2 level sensor. RST2 level sensor is placed at level
which will confirm the availability of sewage to treat a batch of sewage.
Signals from both RSTI and RST2 are required to initiate filling process. If
either RST2 or RSTI do not confirm the availability of sewage, the PLC
migrates into a "Hibernation Mode" and remains in hibernation mode till the
availability of sewage is confirmed by RST2. On receiving signal from the
RST2 after the availability, the PLC automatically migrates into the "Auto
mode1' from the "Hibernation Mode1'.
[0043] Before the initiation of the first process step that is 'filling', the PLC checks
the sewage top level in the SBR tank through the level sensor SBR3. If the
SBR3 confirms that sewage level in the SBR tank isnot full the PLC initiates
the filling process. If the SBR3 confirms the availability of sewage up to the
maximum level in the SBR tank, a "Decant Failure" error is displayed; this
has been explained in detail under description of 'Fault Detection and
Mitigation' features of this invention.
[0044] The PLC logic incorporates an option to turn 'OFF' or 'ON' the blower during
the filling process. This requirement to keep the blower 'ON' or turned 'OFF'
during the filling cycle enables the user to control the treatment levels of
nutrients in the treated sewage. The filling process is terminated either by
the signal from the level sensor SBR3 in SBR tank or if the filling time
elapses. Ideally, the filling should terminate by the signal from the SBR3
which will then trigger the initiation of the second step in the treatment
process "the aeration" step. If the filling is terminated by the lapse of filling
time rather than by the SBR3 level sensor, a "Filling Incomplete" this has
also been explained more fully under the description on 'Fault Detection and
Mitigation'.
.
[0045] Before initiating the aeration step, the PLC checks for the discharge of
excess sludge from the SBR reactor. If the process batch number is a
multiple of a user defined de-sludging nth batch, the SBR remembers and
initiates the removal of sludge through an air lift mechanism triggered by a
sludge solenoid valve before the decantation of the treated sewage is
complete. Completing the check for excess sludge removal, the PLC turns
on the blower for the user defined time duration during which the biological
oxidation of organic pollutants occurs. After the time to keep the blower on
lapses, the PLC initiates the third step of the treetment process "the settle"
process.
1 [0046] During the settle process, the blower is turned off and quiescent conditions
are maintained in the reactor to facilitate the settling of sludge granules. The
sludge settles to the bottom of the reactor leaving a clear supernatant of
treated sewage above the decant level in the SBR tank. The 'Disinfection
System' is set to turn on 5 minutes before the settling time elapses. The
step 4 in the treatment process, i.e., "the decant stage" is set to initiate by
the PLC after the user defined settling time lapses.Before the decant
process is initiated, the PLC checks the top level of the treated sewage in
the filter feed tank through the level sensor FFT3. If FFT3 confirms the
availability of treated sewage to its maximum level, the PLC does not
decant the treated sewage from the SBR tank and displays "FFT full" error
and migrates into a fail safe mode which has the same time settings as that
of the hibernation mode.
[0047] The PLC initiates the decant process by draining the treated sewage from
the SBR tank through decanting mechanism. The decant mechanism is kept
on either for user defined time duration or till the level sensor SBR2 turns
signals the PLC to complete the decant process. Ideally the level sensor
SBR2 must turn off the decant mechanism and the disinfection mechanism
but if the decant time lapses before the SBR2 signals termination of decant
process, "Decant Incomplete" error is displayed with audio and visual
signals. If the PLC has marked the batch for discharge of excess sludge,
the sludge solenoid valve is opened for user defined time before the decant
time elapses. The excess is thus removed out of the SBR by an air lift
mechanism.
[0048] After the decant process is complete, the PLC acknowledges the
completion of a treatment cycle and increments the batch number for
records. The PLC then starts the next cycle of treatment to treat subsequent
batches of sewage.
[0049] Blower Operation Logic: The schematic for blower operation logic is shown
in Figure 7. To take effect this logic, one number of pressure settable
pressure switch is installed in the air line header. When a blower is turned
on the PLC will wait for input from the pressure switch. When blower does
not function despite the signal to turn it on, the PLC does not receive
feedback from pressure switch. This prompts the PLC to do:
i. Display "Blower 1 fault" in the display with visual and lor audio alarm
ii. Turn on the standby blower and waits for the response from the
pressure switch.
If the feedback from the pressure sensor is achieved, then the process
continues with the working blower till the faulty blower is rectified. If the
standby blower too fails to trigger a feedback from the pressure switch, the
PLC displays failure of both the blowers with required alarms.
[0050] Filter Feed Pump Logic: The schematic for filter feed pump logic is shown in
Figure 8. To take effect this logic, one number of pressure settable pressure
switch is installed in the water discharge header. When a filter feed pump
(FFP) is turned on, the PLC will wait for input from the pressure switch.
When the PLC does not receive feedback from pressure switch, the PLC
executes the following
i. 1. Display "FFP 1 Fault " in the display with visual and lor audio alarm
ii. Turn on standby FFP and waits for the response from pressure switch
If the feedback from the pressure sensor is achieved, then the process
continues with the working FFP till the faulty blower is rectified. If the
standby FFP too fails to trigger a feedback from the pressure switch the
PLC displays failure of both the FFPs with required alarms.
[0051] Fault Detection and Mitigation: The PLC is programmed to look for failures
during the execution of the process on a continuous basis. Parameters of
air and water pressure are monitored in the pipeline within the sewage
treatment plant. Any drop in pressure is detected in-line and the PLC
executes the fault mitigation codes thereby solving the problem and
simultaneously alerts the operator onsite by a visual and lor audio alarm
and the supervisor located offsite remotely over LAN I GPRS. The built-in
logic for fault detection is described here under.
[0052] Fillinn lncom~lete Fault Logic: The time taken to fill the Sequential Batch
reactor (SBR) Tank is known through the flow rate of raw sewage pump.
The (SBR) tank is therefore equipped with a top level sensor designated as
SBR 3 which turns the Raw Sewage Pump off after the level switch gets
activated when the tank in full. Logicis incorporated here so that the raw
sewage pump will turn off if either the time lapses or when the level sensor
"SBR 3" changes status from close to open, whichever is earlier.
[0053] The time to fill the tank is user defined and a time of "Theoretical time" + 15
Min is punched in as fill time. Theoretical time is calculated - based on
volume to be filled and flow rate of pump. Ideally, under normal operation
condition, the level sensor will be activated before the fill time elapses. If for
some reason, the fill time elapses before the level sensor is activated then a
"Filling Incomplete" error is noted.
[0054] The same logic applies for chocking of pipes and impellers of Dump. If for
some reason the line gets chocked, the filling time lapses before the level
sensor is activated and hence we could alert the operator to check the
reason of incomplete filling which may be chocking of pipes or damage to
Pump.
1 I [0055] Fill Failure Fault: When a batch decants, the SBR tank's intermediate level
sensor (SBR2) will close that triggers processes those terminates the
decant process. So when the filling process starts for the next cycle, the
SBR2 level sensor must open which happens due to rise in water level due
to pumping of fresh sewage that is to be treated. The change discussed in
the status of the SBR2 level sensor must happen within the user defined
time (Time punched in depending on the flow rate of the pump, 10 min by
default). If the change in the status of the level sensor does not happen in
the user defined time then the plc stops the raw sewage pump that is
running and turns on the stand by pump. The PLC also flashes a message
"RSPI Error". The plc waits for the status of the level sensor to change
within the user defined time. If this does not happen even after turning the
standby pump, then plc displays "RSP 2 error" and "Fill Failure" as both
pumps are defective and no filling has happened.
[0056] Decant Incomplete Fault: The 'Decant Incomplete' flash implies that while
decantation process has begun but the complete batch volume is not yet
decanted out of the SBR reactor. The decant phase in a cycle occurs after
settling phase and continues till the intermediate level sensor in SBR tank
(SBR2) closes. So under normal operating conditions, the SBR 2 level
sensor changes the status from open to close when required preset decant
level is achieved. Also, a user defined decant time is punched which is 15
min more than the theoretical decant time, by which time the decant
process is supposed to be over. If for any reason, the user defined decant
time elapses before the level sensor (SBR2) changes status then "Decant
In~ompleteis~ ~fl ashed and noted as the decant is not complete to the
required levels. This warning is propagated so that the operator can be
aware of the fact that the STP is not bound to run to its full treatment
capacity.
[0057] Decant Failure Fault: The 'Decant Failure' flash implies that decantation has
never happened during the decant phase and the SBR tank is full and not
ready to accept the batch. Before the decant phase is initiated by PLC, the
top level sensor in the SBR tank "SBR3" will be in open state which was due
to proper filling operation during the fill cycle. So when the decant phase
starts, the water level in the SBR tank starts to drop and this will change the
status of level sensor SBR3 from open to close. When the level sensor does
not change status, it means that the treated sewage from the SBR tank was
not decanted. The PLC checks the status of the level sensor SBR3 before
the filling cycle of the next batch. If the level sensor SBR3 is still open, the
PLC notes that the decantation did not happen for the previous batch and
displays "Decant Failure" message.
[0058] Process Efficiency and Advantages of this Invention
This invention provides several advantages over that of the prior art as
described below:
i. The implementation of this invention reduces the human presence to a
single operator from a single point of control. In addition, it enables to
synchronize various processes of the plant helping in improving the
efficiency of the system with appreciable cost saving as we\\ as
enhancing the quality of purification process.
ii. Moreover, a system enabled with this invention minimizes the "human
error" provides vital information absolutely timely accompanied by a set
of records and alarms.
iii. This invention also increases the security of the system, and helps
avoid any possible risk of environmental contamination.
iv. The proposed "commissioning logic" enables the end user to recommission
the SBR without the onsite assistance from the technology
provider. This ensures a reliable restart process, avoids time wasted
before starting re-commissioning and reduces environmental impact by
avoiding dumping of untreated sewage before re-commissioning.
v. The "filling incomplete", "Filling failure", "Decant Failure" and "Decant
Incomplete1' logic incorporated in the PLC helps us identify that the
plant is not running to its full intended capacity. These alert
mechanisms help us take evasive actions before the sewage from
collection or equalization tank overflows and inundates the STP area.

We claim:
1. A sewage treatment plant based on sequential batch reactor equipped with
programmable logic controller (PLC) for process control for start-up
commissioning, operation and maintenance in auto mode as well as fault
detection, repairing and re-commissioning as described in this invention.
2. A sewage treatment plant based on sequential batch reactor equipped with
programmable logic controllers (PLC) for process control for start-up
commissioning and re-commissioning.
3. A sewage treatment plant based on sequential batch reactor equipped with
programmable logic controllers (PLC) for automatic control of pre-defined
process parameters of filling and aeration time to allow the plant to run
automatically self-controlled in 'auto mode'.
4. A sewage treatment plant based on sequential batch reactor equipped with
programmable logic controllers (PLC) to be able to detect faults with blowers
and filter feed pumps with running processes and equipment and take evasive
corrective actions of switching on standby blowers and pumps.
5. A sewage treatment plant of claim 4, where in the PLC continuously monitors
the blower operation during the aeration and automatically turns on the standby
blower upon detecting the fault with the main blower and simultaneously gives
an alarm onsite or off-site over GPRS connection through audio or video signal.
. 6. A sewage treatment plant of claim 4, where in the PLC continuously monitors
the filter feed pump operation during the aeration and automatically turns on the
standby filter feed pump upon detecting the fault with the main filter feed pump
and simultaneously gives an alarm onsite or off-site over GPRS connection
through audio or video signal.
7. A sewage treatment plant of Claim 4, wherein the PLC continuously detects the
availability of raw sewage in the raw sewage tank during the start-up
commissioning and can display audio and / or video signal indicating 'Sewage
insufficient' to commission.
8. A sewage treatment plant of Claim 4, wherein the PLC continuously monitors
the operation of raw sewage pumps through filling of SBR tank and sensing the
level of sewage at a given point after a user-defined time lag determined based
on the known flow rate of the raw sewage pumps and can display audio and I or
video signals indicating
a. 'Fill Failure' signifying that both raw sewage pumps have not been
working;
b. 'Raw Sewage Pump 1 Error' signifying that the first pump is not
working; and
c. 'Raw Sewage Pump 2 Error' signifying that the second pump is not
working
9. A sewage treatment plant of Claim 4, wherein the PLC continuously monitors
the filling process of SBR tank by sensing the level of sewage reaching a userdefined
optimum point based on its designed capacity and continues to
the raw sewage reaches the said user-defined optimum level or a user-defined
time period has elapsed and also turns off the raw sewage pump upon these
conditions being met.
20. A sewage treatment plant of Claim 4 wherein the PLC continuously monitors the
decantation process out of the aeration tank of the sequential batch reactor
which starts upon completion of settling phase by sensing an intermediate level
sensor in SBR tank and keeps flashing an alert indicating "Decantation
Incomplete" as long as decantation process out of the sequential batch reactor
is not complete to a user-defined level.
I I. A sewage treatment plant of Claim 4 wherein the PLC continuously monitors the
decantation process out of the aeration tank of the sequential batch reactor by
sensing the optimum level sensor of the SBR tank and flashes a warning signal
of "Decantation Failure" implying that decantation did not begin as anticipated
and the aeration tank of the sequential batch reactor is full and not ready to
accept the next batch.
12. A sewage treatment plant of Claim 4 wherein the PLC continuously monitors the
sewage availability in the SBR tank and in the event of sewage being
insufficient for aeration, it automatically turns the process in 'hibernation mode'
when the blower runs intermittently for short user-defined time periods to
maintain aerobic conditions for the living bacteria in SBR tank.