Kunshan Jinghe Water Plant (600,000 m³/d): How a Lateral Flow Inclined Plate Settler Retrofit Cut Turbidity Below 1.0 NTU — With Zero Added Energy

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Published: 2025-05-28  •  Category: Cases  •  Tags: Tube Settler · Inclined Plate Settler · Sedimentation Retrofit · Drinking Water · Kunshan

SECTION 1

A 600,000 m³/d Plant With an Obsolete Process-and a Turbidity Problem

Kunshan Jinghe Water Plant is not a small facility. With a design capacity of 600,000 m³/d serving a major city in Jiangsu Province, it underwent an advanced treatment upgrade in 2006–2007, adding ozone-biological activated carbon to its conventional treatment train. For years, the plant operated a flocculation grid → horizontal flow sedimentation tank → flotation tank → V-type filter process-a configuration designed when the raw water source, Kui Lake, suffered from seasonal algal blooms that made flotation essential.

Then, in November 2011, everything changed. A Yangtze River water diversion project came online, giving Kunshan a second raw water source. The new "river + lake" dual-source system dramatically improved raw water quality. Algae levels plummeted. And the flotation tank-once critical-became a high-energy-consumption liability. Before the retrofit, it had already been taken out of service.

But decommissioning the flotation tank created a cascade of problems. The horizontal flow sedimentation tank was only 76 meters long (including a 22-meter terminal collection zone), giving it a relatively short hydraulic retention time. Without the polishing effect of flotation, settled water turbidity drifted up to 1.0–2.0 NTU, and operators could visually see fine floc particles streaming toward the filters. The downstream V-type filters took the hit-shorter filter runs, more frequent backwashing, higher energy and water consumption.

The plant needed a retrofit. But with a clear water tank built directly underneath the sedimentation and flotation basins, and the structure already over 20 years old, they couldn't just tear things down and start over. Every modification had to respect the existing structure, avoid adding weight, and-critically-never risk damaging the clear water tank roof.

SECTION 2

The Design Brief: Retrofitting Within a 20-Year-Old Structure

The engineering team set three clear constraints:

1. Minimize structural damage to the existing tanks-these were built in the 1990s and early 2000s.

2. Pilot on Phase II (50,000 m³/d)-a single treatment train-before considering full-plant rollout.

3. Target: sedimentation tank effluent turbidity stably below 1.0 NTU, with controlled investment cost.

The core strategy was elegantly simple: repurpose the existing flotation tank space as a secondary sedimentation zone, compensating for the horizontal flow tank's insufficient length. But two critical design decisions would determine whether the retrofit succeeded or failed.

SECTION 3

Two Make-or-Break Decisions: Collection Troughs and Settler Type

Decision 1  Keep or Remove the Collection Troughs?

The horizontal flow sedimentation tank had five collection troughs at its end-each 22 meters long, reinforced concrete with stainless steel adjustable weir plates. Keeping them would save demolition work. But the upward flow pattern as water entered the troughs would lift fine floc particles that had barely settled, undermining the entire sedimentation process.

Decision: Remove the troughs. The 22-meter gain in effective sedimentation length far outweighed the additional demolition cost. Water would now flow through distribution holes in the partition wall at a controlled velocity of ≤0.1 m/s, transitioning smoothly into the secondary sedimentation zone.

Decision 2  Tube Settlers or Lateral Flow Inclined Plates?

This was the pivotal technical choice. Both tube settlers and inclined plate settlers use the shallow-depth sedimentation principle-adding multiple parallel settling surfaces to dramatically increase effective settling area within the same tank volume. But the flow direction makes all the difference.

Upward flow tube settlers have four deal-breaking disadvantages in this scenario:

▷ The floc reaching the secondary zone was already fine and light-difficult to settle even in horizontal flow. In upward flow (counter-current sedimentation), water rising against falling particles would carry them upward, making separation unpredictable.

▷ The original flotation tank was only 3.3 m high, with effective depth under 3.0 m after freeboard. Tube settlers need at least 4.5 m for proper lower distribution and upper clear water zones. Insufficient height means poor distribution-the #1 killer of settler performance.

▷ The 400 m² tank area made uniform distribution and collection across a large tube settler field extremely difficult, especially under height constraints.

▷ Any non-uniformity creates local high hydraulic loading zones where fine floc escapes-defeating the purpose of secondary sedimentation entirely.

Lateral flow inclined plates solved all four problems:

✓ Water flows horizontally, particles settle downward-cross-flow with minimal interference. Far more favorable for fine floc.

✓ Horizontal distribution along the 18.6 m tank length provides ample space for uniform flow-no height penalty.

✓ Only 3–3.5 m tank depth required-perfect for the existing 3.3 m flotation tank.

Decision: Lateral flow inclined plates. This was not a close call-the physical constraints of the existing tank made tube settlers non-viable and inclined plates the natural solution.

This is a lesson worth remembering: in retrofit projects, the "best" technology on paper is irrelevant. The only thing that matters is what works within your existing structure's constraints.

SECTION 4

The Final Design: Every Number That Mattered

After the scheme comparison, the retrofit design for the 50,000 m³/d Phase II treatment train came together. Here are the engineering details-the kind you can actually use if you're planning something similar.

4.1  Horizontal Flow Sedimentation Tank Modification

4.2  Lateral Flow Inclined Plate Sedimentation Zone

SECTION 5

Construction: Working Around a 20-Year-Old Clear Water Tank

Construction started in December 2023 and the system was commissioned by mid-January 2024-just six weeks for a retrofit on a live treatment train. Three challenges stood out:

Challenge 1  Drilling Through Structural Walls

Both the outlet partition wall of the sedimentation tank and the inlet wall of the flotation tank needed distribution holes. But these walls also served as foundations for the roof support columns. Drill too many holes or make them too large, and you compromise structural integrity. Make them too small, and you get jetting instead of uniform distribution.

After consulting structural engineers, the solution was 16 circular holes of 600 mm diameter in each wall-few enough to preserve strength, large enough to achieve the target 0.1 m/s distribution velocity without floc shear. Sometimes the best engineering decision is knowing when not to optimize further.

Challenge 2  Don't Break the Roof (It's Also the Floor)

The flotation tank's bottom slab is the clear water tank's roof. Any damage would mean leakage into the treated water storage below-a catastrophic failure mode for a drinking water plant. Every demolition and construction activity had to be planned around this constraint. No exceptions.

Challenge 3  Weight Neutrality

You can't add load to a roof that wasn't designed for it. The retrofit had to be weight-neutral: every kilogram of new equipment installed had to be offset by kilograms removed. The engineering team accounted for this meticulously:

✓ Removed: flotation equipment, skimmers, pipelines, brick partition walls

✓ Added: ABS inclined plate units (density close to water-near-neutral buoyancy when submerged)

✓ Added: Lightweight concrete sludge slopes (density matched to water, avoiding dead load)

The weight-balance approach is a critical lesson for any plant considering a tank-top retrofit: material selection isn't just about chemical resistance and cost-density matters when you're working on top of another structure.

SECTION 6

Results: Turbidity Below 1.0 NTU-Without Adding a Single Watt

The retrofit was commissioned in January 2024. Five months of operational data (February–June 2024) confirmed the design met its targets:

Here's the part that matters most to plant managers: this entire improvement was achieved without adding any form of energy consumption. The inclined plate sedimentation zone operates entirely by gravity. Compare that to the flotation tank it replaced-running air compressors, recirculation pumps, and skimmer drives 24/7. The retrofit didn't just improve water quality; it eliminated a major OpEx line item.

One operational issue emerged that's worth noting for anyone planning a similar retrofit. Before the modification, occasional chemical foam or floating scum at the end of the sedimentation tank would simply fall into the collection troughs and flow to the filters. After the retrofit, the new distribution holes sit below the water surface-surface scum can't escape and accumulates at the tank end. It doesn't affect effluent quality, but it's unsightly. The plant currently handles it with manual removal and may add surface skimming equipment later. Small detail, but the kind that makes operators' lives easier or harder.

Figure 1  Retrofit Layout: Horizontal Flow Sedimentation Tank + Lateral Flow Inclined Plate Sedimentation Zone (Phase II, 50,000 m³/d Train)

Figure 2  Inclined Plate Units Inside the Repurposed Flotation Tank - 2 Units, ABS Material, 80 mm Plate Spacing

SECTION 7

What This Project Teaches Us About Sedimentation Retrofits

This project is not a one-off. Plants worldwide face the same situation: an old process unit that once served a purpose, now consuming energy and delivering marginal value, sitting on valuable tank real estate that could be doing more. The Kunshan retrofit offers five transferable lessons:

1. Lateral flow beats upward flow for fine floc. When your particles are already small and lightweight, don't fight gravity-use cross-flow separation where water moves horizontally and particles settle vertically with minimal interference.

2. Distribution uniformity makes or breaks settler performance. Whether you choose tubes or plates, if your inlet distribution is uneven, you'll get localized high-velocity zones where particles escape. The Kunshan team obsessed over this-16 carefully sized holes, inlet stabilization zones, outlet stabilization-and it paid off.

3. Material density matters in tank-top retrofits. ABS plates with near-water density and lightweight concrete sludge slopes kept the retrofit weight-neutral. This isn't just an engineering footnote-it's the difference between a feasible project and one your structural engineer will reject.

4. Removing energy consumption is as valuable as improving effluent. The flotation tank's compressors and pumps were a constant OpEx drain. The inclined plate settler runs on gravity. Over a 20-year remaining plant life, the energy savings alone likely exceed the retrofit cost.

5. There's no universal template. What worked in Kunshan-converting a flotation tank into an inclined plate settler-may not apply to your plant. Different raw water, different tank geometry, different constraints. The methodology (evaluate, compare, pilot, verify) is universal. The specific solution isn't.

For plants considering a similar retrofit, the inclined plate (lamella) approach offers a compelling value proposition: add effective settling area without adding footprint, improve effluent quality without adding energy, and repurpose obsolete tankage without major civil works. With plate spacing typically 50–80 mm for drinking water applications and materials ranging from PP and PVC to ABS and stainless steel, there's a configuration for virtually any existing tank geometry.