Evaluation of Treatment Process of 17.3 MLD Tukucha Khola WWTP in Nepal
(1) Plant Components:- The plant components specified in the Contract and the compliance are compared in Table
1 below. The plant components are specified in Section 6 – The Employer Requirements and Annex 5 of
Section 6 of the Basic Design Report (Section 4.3 Purification Components, Section 4.4 Treatment Sections).
The Contractor provisions were verified using Drawing No 815 129 PEE 700 L 001 00 Rev R0.
(2) Notes:
a. The verification in Table 1 is mainly to confirm the inclusion of the treatment component as per the
ERQ but not to confirm its compliance with the specification.
b. The no compliance does not mean that the performance objective has not been met since it is a
design-build plant. The comments require the Contractor to justify/clarify his proposal, which may or
may not be accepted.
Table 1: Comparison of Tender Requirements and Compliance by the Contractor
General
Contract Requirements
Contractor Provision
Remarks
Channel/pipe from Tukucha Khola to
the site of the WWTP
Interceptor channel provided in the
drawing
Interceptor channel/pipe details are
not provided. It is not required for
process design evaluation; However,
control of flow to the WWTP should
be necessary for its proper operation
Debris removal mechanism for water
released to Bagmathi Khola from
Tukucha Khola
Not shown in the drawing
Not required for process design
evaluation;
Flow measuring in Tukucha Khola is
required to establish treated and nontreated volumes
Not shown in the drawings
Not required for process design
evaluation;
The WWTP and Sludge processing
are on two sides of the Bagmati
River, and a syphon connects them
The Contractor proposes the
connection via a bridge
Consider as a more convenient
option than a syphon considering the
flooding and construction
The system should be built to be for
the operational safety of 50-year
flooding
Not included in the process design
reports
This is important to civil design.
However, the hydraulic profile, static
head, pump selection, etc., can affect
the decision to raise the groud. Also,
the SBR unit top slab is to be turfed
and accessible to people
Pre-Treatment and SBR System
Contract Requirements
Contractor Provision
Remarks
Acceptance channel with a stone trap
(one unit)
Not included in the drawings
ERQ requires the stone trap; whether
a provision for future connection of
trunk sewer should be provided now
shall be decided
Automatically raked coarse screen
with washing and compacting of the
screenings (Mechanical) (two units)
Two mechanical Coarse Screen both
operating basis are provided
Provided in accordance with ERQ
Coarse screening (Manual) (one unit)
One unit is provided
Provided in accordance with ERQ
Conveyor stretching course screen
channels
The Contractor proposed one unit of
screw conveyor/compactor
The Contractor complies with the
requirement;
Screening bins (two units)
Contractor two containers of 1m3
each
The Contractor complies with the
requirement; However, 1 m3
container when full will be difficult to
lift and manage manually
Sewage inflow lifting pumps
(operation mode 3+1), including
piping and fittings (valves, non-return
valves)
The Contractor provided four pumps
on three working one standby basis
Provided in accordance with ERQ;
Valve chamber provided
Pumps are centrifugal submersible
ERQ requires pumps to be VFD type
Valve arrangement shall be verified
with mechanical/ piping design
Flowmeter type MID (one unit)
Not provided in the drawing
Required as per the ERQ
Compact pre-treatment units,
including fine screen and aerated grit
trap and floated grease collection
channel (three units)
Fine screen station is provided with
three units on two working one
standby basis; containers are also
provided; oil and grease collection
will take place at grit channels
Compiled with the ERQ; The 1 m3
container, when full, will be difficult to
lift and manage manually;
Grit washing and classifying (two
units)
Three units are provided, including
the standby units
Compiled with the ERQ
Automatic valves for the distribution
of the wastewater to the SBRs (four
units)
The Contractor provided a
distribution chamber with sluice
gates;
The distribution arrangement is
unclear; whether the sluice gates are
automated (actuators) is also not
mentioned. Four units are based on
four SBR units
Two distribution chambers are
provided as the SBR units are built
as twin units
SBR (four units)
Four units are provided
Compiled with the ERQ
Fine bubble aeration system
Four working units are provided with
Compiled with the ERQ
Contract Requirements
Contractor Provision
Remarks
(membrane tubes), including air
supply pipes (four units)
one standby
Mixer (minimum 2 / SBR)
Eight mixers are provided, two in
each tank
Compiled with the ERQ
Excess sludge pump (minimum 2 /
SBR)
Two excess sludge pumps are
provided per tank, one duty and one
standby basis
Compiled with the ERQ
Purified water decanters (2 / SBR)
The proposal seems to be one
decanter fixed or floating per each
tank
The ERQ requirement for two
decanters is not met
Outflow piping with all fittings
Not provided the details
Nor required for process evaluation.
Can be included in the mechanical/
piping design
Effluent flowmeter (MID) (0ne unit)
Not provided in the drawing
Required as per the ERQ
Piloting equipment, instrumentation
The Process Description Report
provided only limited information on
process control
The Contractor should submit a
description of the process control
together with the Process,
Instrumentation Diagram (PID) for
review by the instrumentation
Engineer
level meters, dissolved oxygen
probes, temperature probes, pH
probes
Details not available
Details can be reviewed later and will
not affect the current review of the
process design
automatic samplers (inflow, outflow)
Details not available
Details can be reviewed later and will
not affect the current review of the
process design
Blowers with variable speed, FU
piloting for oxygen demand and
water level depending on the
production of compressed air (1 per
SBR with the possibility for the cross
supply of the SBRs (four units)
Four working units with one standby
are provided
The number of units is Ok; it is not
defined whether blowers are VSD
Each blower with noise reduction
hood
Not Defined
Required as per the ERQ
Associated pipework and ancillary
equipment, valves, fittings
Not Defined
Nor required for process evaluation.
Can be included in the mechanical/
piping design
Contract Requirements
Contractor Provision
Remarks
Outflow lifting station
Purified water pumps, including
piping and fittings (valves, non-return
valves) to lift the treated water out of
the WWTP if gravity flow is not
possible in case of a high water level
of the receiving water, the Bagmati
River
The Contractor provided a sump and
pumping system in the three working
and one standby basis
Compiled with the ERQ
Sludge Treatment Facilities and Biogas Processing
Contract Requirements
Contractor Provision
Remarks
Sludge storage basin (two units)
Only one unit provided
Not compiled with the ERQ
Mixers in the storage basins
(minimum 1 / storage basin) (two
units)
Not clear whether the mixers are
provided in the storage basin
Not clear from the Contractor
proposal
Sludge Thicker; The ERQ is not
specific about the sludge thinker
requirement; It is possible to assume
that requirement could be four units,
one working and one standby basis
for the two basins
Provided one working and one
standby basis
Not compiled with the ERQ, but in
line with the Contractor's proposal
Sludge pumps (to lift the thickened
sludge to the buffering basin on the
site of sludge manipulation (minimum
1 / storage basin) (two units)
Two pumps are provided, one
working, one standby basis
Complied with the ERQ but in line
with the Contractor's proposal
Sludge storage and buffering basin
(one unit)
Provided
Compiled with the ERQ
Mixer in the storage basin (one unit)
Mixer Provided
Compiled with the ERQ
Excentric sludge pumps to draw the
sludge from the storage basin and lift
it through the heat exchanger to the
digesters
Four screw pumps on two working
and two standby basis are provided
Compiled with the ERQ
A tube heat exchanger (to raise the
temperature of the sludge that is
Provided
Section under Digester described the
pumps as excentric screw type;
Provided but not specifically
mentioned, the type of heating
arrangement; but not necessary for
Contract Requirements
Contractor Provision
diverted to the digesters)
Remarks
process evaluation
Digesters with all relevant equipment
like the mixer, gas hood, supernatant
overflow, piping (internal piping for
the injection of excess sludge and
drawing of excess sludge at different
levels (two units)
The contractor proposal is to based
on Passavant-Rodiger's digestion
process, and two units will be
provided
It seems to comply with the ERQ as
the Contractors written statement
confirms that all necessary
components of the Digester will be
provided
Piping for the evacuation of digested
sludge from the digesters to the
digested sludge storage basin
Provided but detailed can be
reviewed only with mechanical/
piping drawings
Complied with ERQ; not required for
process evaluation
Sludge dewatering units (e.g.
centrifuges)
Provided centrifuge units
Complied with ERQ
Polymer station for the production of
polymer solution and polymer adding
to the sludge (two units)
Provided by the Contractor
Complied with ERQ
Excentric screw pumps for the
conveyance of digested sludge from
the digested sludge storage basin to
the dewatering units (e.g.centrifuges)
(three units on a 2W+ 1Stby basis)
Provided
Complied with ERQ;
Conveyor to transport the dewatered
sludge from the dewatering units to
the sludge collection containers
Provided
Complied with ERQ;
Biogas drying and desulphurization
equipment (two units)
A Scrubber unit is provided; Only one
unit is included; However, two
desulfurization units provided
Not complied with ERQ as no
standby is proposed for the scrubber
unit; however, it does not affect the
process evaluation
Biogas holder for the storage of
biogas produced within half a day
(one unit)
The biogas holder provided
Complied with ERQ; not required for
process evaluation
Biogas burner to flame the biogas
that cannot be stored (one unit)
Biogas flare is identified in the
drawing; details of the flare are not
provided
Complied with ERQ; not required for
process evaluation
Generator for the combustion of
biogas and production of energy (one
unit)
Power Generation Room is provided;
Generator details are not included
Complied with ERQ; not required for
process evaluation
(3) Comments on Flow, Influent Quality, Waste Loads and Treated Effluent Quality
a. Daily average and peak flows used in the design are in accordance with ERQ
b. ERQ provided influent quality for BOD, COD, TSS, TKN, NH4-N, and Ptotal the values used in the
design are compatible; in addition the design report included inflow quality for VSS. It is assumed
that VSS was computed on the same basis used in the ERQ to compute wastewater characteristics
and acceptable.
c. The design minimum and maximum temperature are in accordance with the ERQ
d. Waste loads; although some parameters have very small deviations to the values provided in ERQ.
The values used in the design are acceptable.
(4) Purification Requirement
a. The purification requirements of water quality parameters are acceptable except the higher pH
value should be corrected to 8.5 (no impact on process design calculation in this case)
b. There are minimum purification efficiencies provides as percentage of influent quality which should
be checked in the design; In this case the TOC level should be estimated. The ERQ requirement is
The Works must comply both with the limit values and with the minimum purification efficiency
standards specified in Table 3 in ERQ.
(5) Upgrade Requirement
a. ERQ requires that the Contractor to ensure that its layout includes provision for the Employer to
upgrade the WWTP for nutrient in the future (residual volume in the SBRs) although no preparatory
works have to be done with this project.
b. Accordingly it is necessary to illustrate in the layout arrangement / land utilization that the enough area
is available within the allocated land for WWTP is available to implement nutrient removal or that the
designed SBR units can be adopted for nutrient removal
(6) Required Sludge Quality
The design values set for stabilized and dewater sludge is Dry Solids concentration 25% and Loss of Ignition
60%; The ERQ requirements are Dry Solids concentration ≤ 25% (acceptable) and Loss of Ignition ≤ 55% (not
acceptable; as it is a schedule of performance requirement)
(7) MPS Inlet Chamber Dimensioning
Reviewed and no objection. It is good if bid more details on how the dimensions are arrived at is explained.
For e.g. the Length = 2 x width of mechanical screen channels + width of manual screen channel + 2 x width of
walls separating the channels
Water depth is taken as 1.5 meters it should corresponds with the inflow arrangement from Tukucha Khola.
Details of inflow arrangement from Tukucha Khola are not provided yet.
Note: the following comments are prior to my review of the hydraulic calculations. It is possible some of
these comments are covered/ explained in the hydraulic design calculations.
(8) Trash and Course Screen Channels (Mechanical)
The ERQ design criterion requires (Annex 4 Section 4.65.5 Table 10) the clear spacing between bars to be less
than 10mm. In the design the spacing used is 20mm.
Water depth considered is 0.70 meters. However, the MPS inlet chamber water level is 1.5 meters. It seems that
you have planned the screen channel bottom to be 0.7 meters below the top water level of the Inlet chamber
and allow very mild slope to the screen channel. I have checked the flow velocities and other hydraulic
parameters for the screen channel based on this assumption. However, I could not but get the values computed
in the design report. Provide the following information.
(a) Hydraulic arrangement between the Inlet Chamber and the channel (the profile).
(b) I suggest to use specified bar spacing in the ERQ unless there can be reasonably good argument for using
20mm; 15-20mm is the values usually adopted in sewerage systems but in this case they may have
proposed 10mm to protect the fine screens more than the pumps.
.
(9) Trash and Course Screen Channels (Manual)
The ERQ design criterion requires (Annex 4 Section 4.65.5 Table 10) the clear spacing between bars to 20mm.
In the design the spacing used is 30mm.
The description provided for mechanical screens apply.
(10) Screens, Conveyor and Compactor
These aspects will be probably reviewed by the mechanical engineer as well. I have no objection. The main
concern is the size of the container it would be very heavy and difficult to handle. The ERQ specified
arrangement/mechanism for lifting but large size containers are very difficult to handle unless EOT crane and
can lift and unload to the collection truck directly. .
(11) Screens Channel
The width of the screen channels 1.2 meters (mechanical) and 1.7 meters (manual) and length of the channel
5.1 meters would be adequate despite verifications requested in (8) and (9). However, channel bottom may
require level adjustments such as small drop at the downstream of the screen and rise at the end to compensate
for free fall to pump sump (if free fall is allowed)
(12) MPS Sump
The review of pump sump and pump selection are as follows.
(a) The pumps are selected to remove maximum hourly flow (accepted)
(b) Accordingly three pumps with 720 m3/hr capacity are provided which allow pumping out of 2160 m3/hr
(c) The effective volume of sump is 137.5 m3. Accordingly it takes only 3.8 minutes to empty the tank and
another 3.8 minutes to fill the tank. Thus total cycle time will be 7.6 minutes.
(d) Accordingly to above the pumps will switch on and off approximately eight times per hour. (CPHEEO
recommendation is 6 on/off cycles per hour). This means little bigger effective volume would be required.
(e) The pump sump size is also depending on pump arrangement. It is necessary to provide space between
pumps and to provide space for smooth supply of flow to prevent turbulence and inflow of water with
entrapped air. It is necessary to use Guidelines by Hydraulic Institute (HI) or manufacturers in this case. I
assume that there should be IS standards. However, HI guidelines are used in India as well. Note: May be
you have already considered these aspects in sump sizing however as the design report does not indicate
such information I have provided these comments.
(f) As I understood the pumps provided are fixed speed. The ERQ however ask for one Variable Speed Pump
(Section 4.10 Annex 4 ERQ). In my opinion in that case there should be two fixed speed pumps and two
variable speed pumps each can deliver 720 m3/hr maximum.
(g) Refer to the section 4.10 for the described pump operating philosophy. The proposed approach in the ERQ
is to optimize the energy consumption.
(h) Refer to the comments and reconsider the pump sump sizing and pump selection.
(13) STP Inlet Chamber (I assume this means the distribution chamber to fine screens)
Review with the fine screen channel
(14) Fine Screen Channel
a. Number of fine screens are provided in accordance with the ERQ
b. Manufacturers catalog is required to verify the other calculations of the design
c. It is not clear of inflow arrangement (internal/ external) and solids removal arrangement and the
controlling flow and velocities; flow splitting arrangement to the three fine screening chains. These
information is likely to be in the manufacturers installation drawing/ broacher.
d. ERQ requires the screen matter to be disposed in endless plastic bags in containers. The design
proposal only provides containers. If the design wishes to deviate from the ERQ it can be considered
with justification
(15) Grit Chamber
a) ERQ requires fine screens, and grit removal (including oil and grease removal in the same channel.
Accordingly three separate channels are provided. The proposal is compatible with the ERQ.
b) The return flow is calculated using mass balance; Could not locate the mass balance calculation.
c) The design criteria listed in the document are partly from the ERQ; however, there are additional design
criteria and source of which is not provided.
d) The details provided in the design report indicates that the Contractor plans to construct the aerated
integrated grit removal system at site and incorporate the necessary equipment such as blowers/ air
compressor, grit pumps, grit clarifier etc. The information provided for the design is not adequate for the
review of the design. Following documents are required for the design review.
i. Detailed drawing of the integrated grit and oil/grease removal system
ii. The drawing shall include the oil and grease removal (skimming arrangement), grit
removal arrangement
e) The review of mechanical units will be covered by the Mechanical Engineer in addition.
(16) Quantification of Screen matter, Oil Grease and Grit Volumes
a) The selection of equipment such as grit classifiers, screw conveyor for screens are based on quantity
of screen, grit matter removed. The source of data is mainly the Metcalf and Eddy which mostly
relevant to US and Developed Country situations. Therefore it would be better if we based the estimate
on Nepal/Indian or developing country conditions/ situations. Grit quantities and screen quantities are
high in developing countries due to poor O&M of systems, interconnection of sewer and storm water
systems and poor solid waste management.
Process Review SBR Plant
(17) Distribution Box
a. Average and Peak Flow calculation accepted
b. It is good that with each unit to briefly explain how it operates. In this case how the wastewater is fed to
the SBR reactor; that would enable understand clearly the design parameters and values depicted in
the tabular form
c. Proposed design values are acceptable
(18) SBR System
a. Average and Peak Flow calculation accepted
b. Design Wastewater Characteristics – following comments should be considered
i) Abbreviations should be defined as some abbreviations are not commonly used
ii) Assumptions were made in estimating certain parameters such as sBOD, rbCOD it is necessary to
provide justification or if the Contractor has carried out analysis of effluent samples then provide
support evidence
iii) pH in the wastewater analysis is provided as 6.5-7.5 and in the treated effluent as 6.5-8.5; however
in SBR design the pH is taken as 5.5-9.0; Is there any reason for wider pH range at this stage; the
pre-treatment processes are not affecting pH (as I see it)
c. Desired treated effluent characteristics values taken for the selected parameters are correct
d. Ambient Conditions – following are the comments
i) Temperature values taken are in accordance with ERQ
ii) The elevation is taken as 1700 m MSL but as per the report the evaluation is about 1280 m MSL
e. c