ADVANCED DIGESTION – FROM CONCEPT TO COMMISSIONING

Powell, C. and Plaza M.

GHD, United Kingdom

Abstract

A review and discussion of the design processes and challenges encountered to develop an advanced

digestion project at an existing WwTW from client concept through tender stage, detailed design,

construction and commissioning. Managing change within the design and the implications for

programme and costs

Following the client’s decision to proceed with an advanced digestion solution sized for 11.5tds / day,

and to provide an enhanced product, GHD were engaged by the Framework Construction Partner as

Civil & MEICA designers to ensure that the proposed solution could be engineered, designed and

constructed to overcome a number of challenges;

• Regulatory date to be achieved.

• Less detailed client scope document than previous projects.

• A novel, containerised Thermal Hydrolysis Plant.

• A restricted site footprint, congested with operational infrastructure above & below ground.

• An existing sludge digestion and dewatering process to be re-engineered for changing sludge

consistency, composition and rheology.

• Existing assets that the client wished to reuse / convert.

• Management of additional liquor flows, solids and nutrient load.

• Management of changes to scope, client requirements and design elements.

Under the clients’ procurement model, the client provides a basis of design, a project scope and takes

process responsibility for the proposed solution and process throughputs. Leading the detailed design

from front end mass & energy balances, to construction stage MEICA & Civil design with extensive use

of 3D modelling, GHD ensured that robust, cost effective design was delivered on time. GHD were

responsible for specifying and integrating numerous vendor packages into the scheme including the

THP plant & steam boiler, new sludge thickening & storage, existing CHP system, Ultra Violet (UV)

Disinfection of Final Effluent service water, Dissolved Air Flotation (DAF) clarification for liquors

treatment and chemical dosing.

Effective collaboration between stakeholders was achieved with regular design reviews, site inspections

and participation in the detailed HAZOP / ALM process ensured safe construction, operation and

maintenance.

Keywords

Thermal Hydrolysis, thickening, liquors, dilution, 3D, compact, change, programme

Introduction

The project comprised the installation of a new Thermal Hydrolysis Plant (THP) plant at a large WwTW

(P.E. 100,000) with the main driver to produce an enhanced digested sludge cake by a set regulatory

date.

The THP plant selected by the client was a CAMBI B2-4 modular, skid mounted system, believed to be

the first installation of this type in the UK. In addition to the THP plant, new sludge processing assets

were required and some existing assets converted to accommodate the works.

The capital value of the project was approx. £20million and the initial construction programme as follows

• Client scoping design 2013

• Tender design Dec 2013 – Apr 2014

• Order placed with construction contractor Jun 2014

• Design start up July 2014

o Delete air blast cooler July 2014

• Coding issue MEICA design Aug – Dec 2014

• Coding issue civil design Jan – Mar 2015

• Start on Site Sep 2014

o Change design for 3% s/s raw sludge Apr 2015

o Delete UV for dewatering Apr 2015

o Layout changes Mar 2015

o Delete LTP, add DAF Unit May 2015

• Wet Commissioning & start up Dec 2015

• Project In use Jan 2016

Discussion

Existing Sludge Treatment Process

The WwTW is a conventional ASP treatment site with tertiary ammonia reduction via Nitrifying Trickling

Filter (NTF) and serves a PE of approx. 100,000.

Prior to the project, and as per Figur, co-settled primary & secondary sludge’s were screened before

being transferred to a sludge storage tank. The screened sludge was pumped to the 2 No. primary

mesophilic digesters with heating provided by waste heat from a CHP engine and a backup propane

boiler if required. Following primary digestion, sludge flowed under gravity to 4 No secondary digesters

for a further 22 days aerobic treatment to inhibit biogas production. Sludge’s were then pumped to

smaller storage tanks prior to final dewatering via centrifuges with conveyors placing the final cake in

the cake storage area before transport offsite and disposal onto agricultural land.

Figure 1: Existing Process Flow

Modified Sludge Treatment Process

The main changes to the existing process are as follows and shown in Figur

• Pre-thickening of the existing co-settled sludge from 2-3% to 16-20% via new decanter

centrifuges.

• Storage of thickened cake in a new 400m3 silo

• Installation of the pre-treatment THP process & steam boiler

• Modification of existing CHP engine to divert exhaust to a waste heat recovery boiler and jacket

water to heat exchanger.

• Post THP Sludge cooling to avoid excess temperature within digester.

• Decommissioning of 1No digester leaving 1No in service.

• Decommissioning of 4No secondary digesters

• New, intensive de-gassing process with air saturation via low pressure blowers.

• Thickening process centrate liquor balancing and return

• New dewatering liquors liquor treatment via DAF.

• New Odour Control Unit for sludge process

Figure 2: Modified Process Flow

Design process & programme

Initial Engagement & Tender Stage Design Services

Following appointment by the Framework Construction Partner, GHD undertook a series of meetings

with the client and their process consultant to discuss the agreed solution and review the initial tender

/ scoping design.

The client had completed an outline design consisting of basic P&IDs, mass balance / PFD, unit sizes,

a basic electrical single line diagram (SLD) and a proposal for the area of site to be used for the new

project (see Figur) . GHD were required to develop the design sufficiently to allow the contractor to

accurately price the project as per the terms of their contract with the client.

One of the biggest challenges at this point was the small footprint available within the existing works;

there was a large amount of equipment to add within a very confined site. Other significant challenges

were constructability and sequencing of the build and ensuring that the existing site operations weren’t

unduly affected. The novel nature of the selected THP unit was also a consideration.

To address these issues, GHD proposed that most effective way to develop this project was to

undertake as much design as possible in 3D software as this would allow rapid updates and allow walk

through visualisation during review meetings, allowing all parties to see and understand the design

development. This approach combined with weekly reviews with the contractor and the client also

ensured continuous HSEQ reviews with regard to access and safe operations.

Using our experience and examples from previous projects, we successfully provided tender stage

design deliverables including;

• Initial 3D model of the project (see Figur)

• Civil & mechanical layouts of the major plant areas and equipment.(see Figu)

• Estimates of piling and RC requirements

• Tender P&IDs giving details of the equipment, instruments and valving required.

• Pump and equipment sizing for all process items

• Detailed SLD’s and SCADA / PLC control systems

• Control Philosophy

The tender design was completed within 12 weeks and permitted accurate tender pricing and early

supplier engagement. After a short period for internal client approval, the tender stage design was

agreed by all parties and following contract exchange, the project moved to the detailed design &

delivery phase.

Figure 4: Site layout provided by client

Figure 4: GHD Developed Site Layout for Pricing – note additional detail, connections and

unit sizing

Detailed Design Phase

As is essential at this stage of a large, technically complex project, we worked with the contractor to

prepare a design & construction programme sequencing the work to allow identification of the key

design issues and programme milestones that would influence how GHD would provide suitable and

timely design to the contractor.

Figure 5: Extract from 3D model showing thickening and THP area

Figure 6: Thickening area following construction

Key Design Challenges & Issues

Programme & need for accelerated design phase

The key driver for the project was the need to meet a regulatory date for achieving enhanced sludge

cake hence programme was a key challenge throughout the design and construction phases. GHD

worked with the contractor to produce a detailed programme which identified a number of items that

required early M&E design to allow subsequent civil design and / or sub contract procurement;

Early review of bore hole data had identified that ground conditions in the area allocated for new

equipment was of poor quality and structures supporting large loads would require piling; mechanical

and civil design of these items was required within a tight time frame and was achieved by GHD working

collaboratively with the contractor and potential vendors; centrifuge support platform, thickened sludge

storage silo, pipe bridges and degassing tank design progressed quickly to allow groundworks to be

started as per programme requirements. Mechanical & civil layouts, load calculations, structural design

and piling design were all provided within a number of weeks to allow the site team to procure the piling

works.

Following completion of the early stage civil related design, numerous long lead items with GHD’s scope

required detailed process and electrical design to ensure that procurement could start to meet

construction programme; High Voltage Transformers & switching gear, Motor control centres (MCC),

sludge thickening centrifuges, large capacity progressive cavity (PC) pumps, silos and tanks were

specified, laid out and sized.

GHD’s extensive capabilities and experience with these types of equipment meant that the contractor

benefitted from rapid design with minimal need for lengthy TQ processes. In the case of the PC pumps,

early stage vendor involvement verified the challenges of pumping thickened sludge at 20% solids w/w

into the storage silo; system duties of 13m3/hr at 25.5 barg had significant implications for the 250NB

pipework system. MCC design was required to allow order placement and also to allow us to size and

locate the control kiosk with a high degree of accuracy.

Novel THP equipment & Interfaces & modifications to CHP & digesters

In order to integrate the new THP system and provide the appropriate process connections, GHD had

to identify and design a number of interfaces to the existing site infrastructure, particularly the existing

biogas and CHP systems. A combination of site survey, laser 3D scanning, review of third party design

documents and face to face discussions with the CHP engine supplier allowed GHD to identify tie in

points, line sizes and materials for the services and show proposed pipework routes within our model

to allow all parties to clearly understand the design concept and scope boundaries. The CHP engine

exhaust was redirected to provide waste heat to the steam boiler and the engine water circuit was

passed through a new heat exchanger system to recover further heat for the boiler feed water system.

The existing biogas cleaning and boosting equipment was modified to supply the new boiler in parallel

with the CHP engine. The complexity of the interfaces are indicated in Figur.

Figure 7: Interfaces between New THP & Existing CHP

A key feature of the move to THP is that the digester feed concentration is increased from 4 to 10%

giving a more than 50% reduction in volume; as a result, only a single digester would be required for

treatment in the future. This also required a re-engineering of the sludge recirculation system and the

need to cool the sludge prior to digestion rather than heating as with traditional mesophilic digestion.

The original design called for an air blast cooler to provide this cooling duty; however, GHD quickly

proposed that this be replaced with a tube type water cooled item using final effluent as the cooling

media in order to significantly reduce power consumption; this also provided heated final effluent which

was to be used for pre THP sludge dilution.

Managing change during design period

However well-defined a projects’ design basis, or however well-developed a design, the requirement to

manage change throughout a project is inevitable. Changes can occur for a number of reasons and for

this paper we can identify 3 main drivers of change; client driven, contractor driven and GHD driven.

Client driven

Client driven change is often associated with significant changes in scope or design basis – usually for

sound technical or commercial reasons as was the case during this project. However, irrespective of

the driver, significant amounts of redesign are often required, potentially extending programme. For

this project the regulatory date couldn’t be moved, hence GHD and the contractor had to assess the

changes and incorporate the impact in a short space of time – something that we were able to do

successfully.

Centrifuge Liquors Management

In THP type processes, the liquors arising from dewatering digested sludge are known to contain

significant concentrations of ammonia (up to 1500mg/l) and this may affect downstream treatment

processes if not managed correctly. At the outset of this project, the client had carried out a process

review of the downstream system and concluded that a dedicated biological liquor treatment plant (LTP)

was required to reduce the ammonia content to a level suitable for blending with the works inlet flows.

Design of the LTP had progressed to the stage where an area of site had been allocated, civils design

begun on tank bases and layouts, power supplies assessed and allocated and vendor submissions had

been reviewed and an order imminent.

Given the significant capital costs of the plant (circa £1million) and the operational costs associated

with the proposed plant (power consumption - 100 kW aeration and potentially 250kW water heater),

the client carried out a further review and risk analysis. The outcome of this final review determined

that the existing biological treatment process had sufficient nitrification capacity to permit the deletion

of the LTP. However, balancing of two liquor streams and a DAF unit for solids removal from the

dewatering liquors was specified and had to be integrated into the design in a short timescale – (see

Figur). An existing sludge holding tank was reused as a new balancing tank and a liquor return system

controlled by works inlet ammonia load was to be introduced.

Figure 8: Compact DAF Unit in place of full biological LTP

Challenges included finding a suitable location for the equipment, providing timely design and lead

time for manufacture. Deliverables prepared by GHD in a short timescale included full P&IDs, process

calculations, MCC designs and control philosophy. Mechanical design followed upon receipt of vendor

information at tender stage. GHD worked with a vendor to provide a compact, safe and cost effective

design, with a temporary unit providing treatment due to long lead time (24 weeks) for the permanent

works. Whilst achieving this significant change was difficult, the benefits to the project were

considerable; the capital cost was approx. 50% of the LTP scheme and power consumption

significantly lower at approx. 45kW.

Change to specified raw sludge solids concentration

During the solution identification & development stage the client based the process design upon raw

sludge solids concentration of 2.0% w/w, and this information was used to produce the mass balances

used to size main equipment items and process flows. Detail design had been completed for vessel

sizing and modelling, equipment sizing and pump duty calculations on this data, and hence motor sizing,

MCC design and power supplies had also been developed and used in detailed layout drawings.

Shortly after the civil site works had commenced, the client advised that the raw sludge concentration

was to be changed to 3.0% w/w following an operational change at the primary settlement tanks. This

change had a significant impact upon numerous items that subsequently required rapid assessment

and redesign. Pump duty calculations, vessel sizing, pipeline sizing all required changing to

accommodate lower flowrates with MCC and electrical design following rapidly. The changes were

made within a few weeks to allow the contractor to go back to vendors with new specifications without

unduly affecting delivery times.

Contractor driven constructability & risk reduction

Following the completion of the initial design stage, GHD and the contractor began to review the

proposed layout from a constructability and cost approach and a number of changes were tabled to

provide some value engineering;

• Combining two proposed kiosks to form a large MCC / polymer dosing kiosk with separate

rooms.

This was a relatively simple change to accommodate, simplified the construction process and reduced

risk in parallel – although the resulting kiosk was a significant size the need to only build a single

basement & foundation in a tight area was a large positive for the project in terms of safer working,

programme reduction and cost savings.

• Retaining an existing building

Figure 9: Retained building (foreground) giving reduced footprint for new installation

Adjacent to the kiosk area was a storage small building (see Figur) which was due to be demolished

and replaced in another location to allow more space for the new process equipment. The contractor

stated a preference to retain this structure to avoid demolition and the need to relocate. This had

beneficial safety, programme and cost implications to the project as a whole and after consultation with

the client, it was agreed that the structure would remain, however this significantly reduced the space

available for the new equipment which resulted in further redesign and a much tighter footprint for the

new centrifuges, UV equipment and cake silo.

GHD Proposed

In addition to changes instigated by the client and contractor, there were several examples of change

proposed by GHD in order to solve technical issues and / or improve efficiency for the client.

Examples include the GHD proposal to use a final effluent cooler as noted earlier and the proposal to

use potable / service water for flushing the final centrifuges rather than install a UV unit. To prevent the

reintroduction of pathogens, final effluent can’t be used to flush the centrifuges, hence UV had been

proposed to provide clean water. However, due to the need to have water available at very short notice

in case of centrifuge shutdown, a UV plant would have been running for extended periods of time,

consuming power and potentially overheating the water circuit. The CAPEX, OPEX and operational

benefits of using potable / service water were examined by the client who approved the proposal.

Key Commissioning Issues

As with most D&B type projects, construction had started with civils underway prior to completion of

M&E design and a discussion of the construction period is beyond the scope of this paper, however, in

general all items progressed as per programme and commissioning began late 2015 as planned.

Although GHD were not involved in day to day commissioning role, regular updates took place between

all parties to ensure safe and consistent progress was maintained. As the commissioning activities were

undertaken, the majority of processes were started and optimised without issue; however, a number of

key operational issues had to be addressed;

• Maintaining 27/7 operation of the THP with intermittent up and downstream operations.

• Consistent raw sludge thickening for storage.

• Discharge from cake storage silo.

• Consistent pre THP dilution.

• Sensor blinding.

To achieve best efficiencies, the THP plant is to be operated 24/7 with a feed solid consistency of 14 -

16%. The sludge thickening plant to increase solids from 3% is intended to operate approx. 8hrs/day

for 5 days week, meaning that a reserve of solids is required to keep the THP plant operating at night

and weekends. In order to store sludge as a cake within a silo, a consistency of 16 – 22% is required

meaning dilution back to 14-16% is required.

Thickening of the raw sludge to storage consistency was achieved after a brief amount of centrifuge set

up as would be expected; centrifuges are normally operated to achieve maximum solids capture, but

for this application a compromise is required – after extensive trials and testing, the correct operating

parameters (poly dose, speed, torque etc.) were achieved and maintained.

Storage of the sludge proved relatively straightforward, however, the discharge of sludge into the THP

feed pumps inlet hoppers proved more difficult to optimise and caused operational issues such as

inability to control flow and over operation of the outlet valves, leading to actuator failure, sludge spillage

and unwanted THP shutdowns. Investigations showed that improved level control within the pump

hoppers (lower ‘high’ level settings) and modified silo outlet chutes (flexible rubber replaced with solid

chutes) rectified these problems and allowed the silo outlet system and THP feed pumps to operate as

required.

Figure 10: Original Silo Outlet (left) and modified (right)

It was also noted that the pre THP dilution system was performing erratically, leading to inconsistent

feed solids entering the THP unit; the system used positive displacement type dosing pumps to add

heated final effluent into the feed pump hopper where an auger mixed the sludge and water into a

consistent mixture. Heated final effluent from the main sludge cooler was used for dilution as previous

projects had indicated to the client that dilution water needed to be heated. Calculating the correct flow

rate of dilution water was carried out via extensive site trials and lab work as online solids measuring

instruments were unavailable for the site operating conditions. A look up table correlating THP pump

speed with dilution water flow was used to provide a control loop. This system provided consistent

solids to the THP unit, however, the dilution water pumps themselves became problematic with frequent

vibration, failure and poor reliability. Eventually the pumps were replaced with modulating valves and

this proved a more robust solution.

A further issue was the tendency for level sensors in centrate and sludge vessels to blind and give

incorrect measurements, over a period of time a number of these instruments where fitted with flushing

systems where heated final effluent was used to regularly flush instruments via a control system of

timers and solenoids as per Figure .

Figure 11: Typical Level Sensor flushing Installation

Conclusions

1. By working collaboratively with the contractor and client, GHD provided a safe, effective and

cost effective design for a challenging installation using novel equipment into an existing works.

2. Managing change throughout the project lifecycle is inevitable and in this project, change

proved beneficial for overall project outcome – positive relationships between client, contractor

and designer are crucial for successful outcomes.

3. Due to our ability to react quickly and decisively, GHD were able to react to change and provide

effective solutions without adversely impacting programme

4. The management of thickening and dewatering operation liquors is critical for overall works

performance. Existing works capacity should be investigated and modelled thoroughly for any

THP project as part of the scoping phase. The clients’ internal design challenge process proved

beneficial and allowed significant cost and operational complexity to be deleted from this

project.

5. Effective Pre-thickening operations are critical and management of storage and transfer of

thickened sludge at 16-22% is challenging.

6. Controlling silo outlet and pre THP dilution are critical to ensure consistent THP operation,

efficient steam usage due to reduced water content and flow / load management due to

improved cycle time.