SWRO Pre-Treatment DAF

Client Overview

A 45 million litres per day (MLD) seawater reverse osmosis (SWRO) desalination plant on the Mediterranean coast. The facility supplies potable water to a coastal city of 180,000 residents and adjacent agricultural irrigation networks. The plant had operated since 2012 with conventional dissolved air flotation (DAF) pre-treatment, but membrane fouling had progressively worsened.

The Challenge

The plant's original DAF system was engineered for conventional seawater with low organic loading. However, seasonal harmful algal blooms (HABs) and increasing transparent exopolymer particle (TEP) concentrations had created a fouling crisis:

• Membrane CIP frequency: Every 30 days (design: 60 days) — consuming 12,000 L chemicals/month
• Pressure drop increase: Normalised differential pressure rising 0.15 bar/week during bloom events
• TEP breakthrough: DAF effluent TEP > 400 µg/L Xanthan eq. (target < 150 µg/L)
• AOM spikes: Biopolymer concentrations peaking at 2,800 µg/L during summer HABs
• Filter run times: Dual-media filters blinding in 8-12 hours (design: 24 hours)

The Solution

Reynolds & Bauhm proposed and was involved in delivering a two-phase upgrade combining hydraulic optimisation and coagulation chemistry improvement.

Phase 1: CFD-Optimised DAF Contact Zone

Reynolds & Bauhm engineers modelled the existing DAF using ANSYS Fluent with multiphase flow (water + air bubbles). The simulation identified:

• Bubble size distribution skewed toward >100 µm bubbles (optimal: 30-60 µm)
• Contact time of only 2.1 minutes (required: >4 minutes for TEP floc)
• Short-circuiting via an unbaffled outlet weir

Reynolds & Bauhm proposed and supervised the design and installation of:

• Revised saturator system: Pall ring-packed saturator with pressure control to 5.5 bar, achieving 70-80 micron bubble Sauter mean diameter
• Extended contact zone: Baffle extension increasing effective retention to 5.2 minutes
• Variable-speed recycle pump: Automated flow pacing from 8-15% of forward flow based on raw water turbidity

Phase 2: Coagulation Optimisation

Jar testing with site seawater identified that ferric chloride at 3.5 mg/L as Fe achieved superior TEP removal versus the existing alum dose of 8 mg/L as Al:

• TEP removal improved from 45% to 78%
• Lower sludge volume (ferric hydroxide denser than alum floc)
• Better performance across pH 7.8-8.3 (seawater natural range)

Results

Operational & Financial Impact

Compliance & Standards

The upgraded pre-Treatment Process maintains full compliance with WHO Guidelines for Drinking Water Quality and EU Drinking Water Directive (2020/2184). Post-DAF silt density index (SDI) consistently measures 2.8-3.4 (target < 4.0), and modified fouling index (MFI-0.45) is maintained below 3 s/L².

Key Learning

TEP and AOM removal in seawater DAF is fundamentally a contact-time and bubble-size challenge, not merely a chemical dosing problem. Conventional DAF designs optimised for suspended solids removal often fail with transparent, neutrally-buoyant biopolymers. CFD-guided hydraulic modification combined with ferric-based coagulation provides a robust, lower-Operating expenditure alternative to granular activated carbon or ultrafiltration pre-treatment.