Water + Wastewater Treatment — Unit Processes, Regulations, Reuse, Desalination

Water + wastewater engineering covers the conversion of raw water sources (surface, groundwater, seawater, reclaimed) into potable supply, the treatment of municipal + industrial wastewaters back to discharge or reuse quality, and the management of biosolids + brines + reject streams. The discipline spans coagulation chemistry + biological reactor kinetics + membrane separation + advanced oxidation + disinfection + regulatory compliance. This note details the modern treatment train, regulations (SDWA + CWA + EU drinking water + UWWTD), specific projects + vendors + capacities, and the emerging reuse + desalination economy. References: AWWA Standards + WEF MOPs (Manuals of Practice) + 40 CFR Parts 130–142, EU Directive 2020/2184, NSF/ANSI 60 + 61 + 372, MWH “Water Treatment Principles + Design” 3rd ed, Metcalf + Eddy “Wastewater Engineering” 5th ed.

1. Water sources

1.1 Surface water

Rivers, lakes, reservoirs. Variable quality (seasonal turbidity, taste-odor, algal blooms — cyanotoxins like microcystin-LR + cylindrospermopsin). Requires full treatment train. Examples: NYC Croton + Catskill + Delaware (4–5 BGD), LADWP Owens Valley Aqueduct + Colorado River Aqueduct, Chicago Lake Michigan intake (Jardine Plant 1.0 BGD), London Thames + Lee (Thames Water).

Intake structures: shore-mounted screens (low-velocity, fish-friendly per 316(b) Cooling Water Intake rule), offshore raw-water tunnels (NYC City Water Tunnel No. 3, 1970–2026 build, 60 mi long, 24 ft diameter).

1.2 Groundwater

Aquifer-pumped via wells. Generally cleaner than surface (filtered through soil) but vulnerable to TCE + nitrate + arsenic + radium + perchlorate + 1,4-dioxane + PFAS contamination. Often requires only disinfection (chlorine or UV) + iron/manganese removal.

Capture zone analysis via MODFLOW (USGS) + GMS Groundwater Modeling System (Aquaveo) + Visual MODFLOW (Waterloo Hydrogeologic).

1.3 Reclaimed water (reuse)

Treated wastewater further polished + recycled. Categories per EPA 2012 + 2017 Guidelines for Water Reuse: unrestricted urban, restricted urban, agricultural (food crops + non-food), recreational, environmental, industrial process, indirect potable (IPR), direct potable (DPR).

1.4 Desalination (saline source)

Seawater (~35,000 mg/L TDS) or brackish (1,000–10,000 mg/L). Reverse osmosis dominant (see §6).

Major SWRO (Seawater Reverse Osmosis) plants:

Sorek 1 + Sorek 2 (Ashdod Israel; IDE Technologies, ~165 + 200 MLD = 624 ML/d combined). Sorek 2 commissioned 2023; world’s largest SWRO at ~ $0.45/m³ (Israel National Water Co tender 2020).
Carlsbad Desal Plant (San Diego CA; Poseidon Water + IDE/Black & Veatch; 189 ML/d = 50 MGD). Commissioned Dec 2015.
Jubail 3 SWRO (Saudi Arabia; ACWA Power + Veolia ~ 2.2 GLD / 600 MGD; commissioning 2024). Largest SWRO in world by capacity.
Magtaa SWRO Oran Algeria ~ 500 MLD (2014).
Ras Al-Khair SWRO (Saudi Arabia; ACWA + Doosan; ~1.0 GLD; commissioned 2014).
Shoaiba SWRO + Yanbu IWPP (Saudi Arabia).
Tuaspring + Tuas SWRO (Singapore Hyflux 2013, restructured 2020 SembCorp).

2. Conventional drinking-water treatment train

The classical “C-F-S-F-D” train: Coagulation → Flocculation → Sedimentation → Filtration → Disinfection.

2.1 Coagulation

Destabilization of colloidal turbidity + NOM (natural organic matter) by trivalent metal salts that hydrolyze + form positively-charged hydroxide species, neutralizing negative colloid surfaces.

Common coagulants:

Alum (aluminum sulfate, Al₂(SO₄)₃·14H₂O) — most common; effective pH 5.5–7.5; produces aluminum hydroxide floc. Dose 10–100 mg/L typical.
PACl (polyaluminum chloride, Aln(OH)mCl3n−m) — pre-polymerized; broader pH range; lower sludge; higher purity. Kemira PAX-XL series, Holland AlphaChem.
Ferric chloride (FeCl₃) — pH 5–8.5; heavier denser floc; better for high NOM. Kemira FerriClear, K2.
Ferric sulfate (Fe₂(SO₄)₃) — similar to FeCl₃ but generally lower performance.
Polymer coagulants — cationic polyamines + polyDADMAC + polyacrylamides used as primary or coagulant-aid.

Mixing intensity: G-value 700–1000 s⁻¹ for ~ 1 min rapid mix (Camp + Stein 1943). G = (P/μV)^0.5, where P = power input, μ = dynamic viscosity, V = tank volume.

2.2 Flocculation

Slow mixing aggregates microflocs into settleable flocs. Camp velocity gradient G typically 30–60 s⁻¹ for 20–30 min. Tapered flocculation: G higher upstream, lower downstream to avoid breaking up flocs.

Tank types: horizontal paddle (Lightnin + Philadelphia Mixing Solutions), vertical turbine, hydraulic (over-under baffles), sludge-blanket (Pulsator + Aquasource).

2.3 Sedimentation

Removes flocs by gravity. Design: Stokes settling velocity Vs = g·(ρp − ρf)·d²/(18μ) for discrete particles; flocculent settling more complex (zone settling, hindered settling). Hazen overflow rate vo = Q/A = surface loading rate; design typically 1.0–2.5 m/h (24–60 m³/m²·d) for conventional, 3–8 m/h with tube settlers.

Tank types:

Rectangular horizontal-flow — length:width 4:1+, depth 3–5 m.
Circular center-feed radial-flow — common in larger plants.
High-rate (plate + tube settlers) — Lamella inclined plates increase effective settling area 5–15× per unit footprint. Brentwood AccuPac + Munters CrossFlow + Meurer Lamella + Hydro-Cellect.
Solids-contact (sludge-blanket) — coagulation + flocculation + sedimentation in single basin; common with Pulsator (Veolia Suez), Aquaflo Reactivator (WesTech), Densadeg high-rate (Suez).
Ballasted flocculation (Actiflo / Densadeg) — Veolia (Actiflo with microsand) + Suez (Densadeg with weighted polymer). Overflow rates up to 80 m/h. Used at NYC Croton (290 MGD; commissioned 2015), London Thames Water plants.
Dissolved Air Flotation (DAF) — air microbubbles attach to floc + lift to surface (skimmed off). Used for algae-laden + low-density-floc waters. Leopold Clari-DAF, Krofta Sandfloat.

2.4 Filtration

Removes residual floc + particulates by passage through granular media.

Media combinations:

Mono-media sand — slow, used at slow sand filters (0.1–0.4 m/h) or pressure sand.
Dual-media (anthracite + sand) — most common; anthracite top (0.9–1.1 mm, 0.5–0.7 m depth), sand bottom (0.4–0.6 mm, 0.2–0.3 m depth). Filtration rate 5–12 m/h (gpm/ft² 2–5).
Tri-media (anthracite + sand + garnet) — deeper bed graded coarse-fine; rates to 25 m/h.
GAC (granular activated carbon) — also adsorbs taste/odor + NOM + micropollutants. Calgon Filtrasorb F300/F400/F600, Norit ROW 0.8 Supra, Jacobi Carbons AquaSorb, Cabot Norit GAC. Service life 2–5 years before reactivation.
Biological filtration (biofiltration / BAF) — biofilm on media degrades BOD + ammonia + assimilable organic carbon (AOC). Kruger Biostyr (Veolia), Biocarbone, Degremont Biofor (Suez).

Slow sand filters — 0.1–0.4 m/h; biological “schmutzdecke” layer; common in EU + small systems. Operational classic at Cheddar reservoir UK, Dordrecht NL.

2.5 Disinfection

Inactivates pathogens (bacteria, viruses, protozoa).

Free chlorine (Cl₂ gas, NaOCl bleach 12.5 %, Ca(OCl)₂ hypochlorite) — most common; cheap; residual maintained in distribution system. Free chlorine pH-dependent (HOCl vs OCl⁻).
Chloramines (NH₂Cl + NHCl₂) — formed by NH₃ + Cl₂; weaker but more stable residual; less DBP formation. Used for distribution-system residual.
Ozone (O₃) — strongest oxidant. Generated on-site from oxygen via corona discharge (Ozonia/Suez VarioStar + ME series, Wedeco OCS + GSO). Half-life minutes. Excellent for taste/odor + iron/manganese + color + micropollutants. Doesn’t provide residual.
UV disinfection — 254 nm + 185 nm UV-C inactivates by DNA damage. Low-pressure (LP) + medium-pressure (MP) + LED. Suppliers: Trojan UV (Suez), Xylem Wedeco, Calgon UV (Evoqua/Xylem), Wedeco LBX/Spektron. Dose typical 40 mJ/cm² for 99 % inactivation of bacteria + viruses (Cryptosporidium 12 mJ/cm² 3-log per LT2ESWTR).
AOP (Advanced Oxidation Processes) — UV/H₂O₂, UV/O₃, O₃/H₂O₂ (Peroxone), Fenton (Fe²⁺/H₂O₂), photocatalytic TiO₂. Generates hydroxyl radicals (·OH, E° = 2.8 V). Used for 1,4-dioxane + NDMA + pesticides + pharmaceuticals.

CT concept — disinfectant concentration C × contact time T. EPA Surface Water Treatment Rule + LT2ESWTR tabulate required CT for log inactivations of Giardia + viruses + Cryptosporidium at various pH + temperature.

DBPs (Disinfection By-Products) — THMs (trihalomethanes) + HAA5 (haloacetic acids) + bromate + chlorite. Regulated by US Stage 1 + Stage 2 D/DBP Rule: TTHM 80 µg/L, HAA5 60 µg/L locational running annual average (LRAA).

3. Advanced drinking-water treatment

3.1 Membrane processes

Process	Pore size / MWCO	Pressure	Removes
MF (microfiltration)	0.1–10 µm	0.1–2 bar	Particles, bacteria, protozoa
UF (ultrafiltration)	0.001–0.1 µm / 10–500 kDa	0.5–5 bar	Viruses, macromolecules, NOM (some)
NF (nanofiltration)	200–1000 Da	5–15 bar	Multivalent ions, NOM, hardness, micropollutants
RO (reverse osmosis)	< 200 Da	10–80 bar	All dissolved species; near-zero permeate

Module configurations:

Spiral-wound — RO + NF + UF. Dominant for high-pressure / saline. Pentair X-Flow + Toray + DuPont FilmTec + Hydranautics/Nitto + LG Chem + Veolia Suez Aquasource.
Hollow-fiber — UF + MF. Most municipal HF membranes are PVDF + PES.
Tubular — small-diameter (10–25 mm) tubes, high crossflow; industrial wastewaters.
Ceramic — Al₂O₃ + SiC + TiO₂; high temp/chemical resistance (Pall Schumasiv, Atech, Liqtech).

Membrane manufacturers (RO/NF):

DuPont FilmTec BW30 + LE-440i + SW30HR LE-440i + SW30XHR-440i + ECO + Eco PRO XR.
Hydranautics (Nitto) ESPA series + SWC series + LFC + CPA series.
Toray TM710D + TM820V + TML20 + Romembra TS series.
LG Chem Water Solutions LG NanoH₂O QFX + BW series.
Veolia/Suez DOW + Pentair X-Flow + Lanxess Lewabrane.

UF manufacturers:

Pall Aria AP-1 + Microza UJL, Suez ZeeWeed 500/700/1500, Asahi Kasei Microza UNA, Inge Multibore (BASF), Toray HFU-2020/2520, Pentair X-Flow Compact 33.

3.2 Ion exchange

Cation IX (sodium-form for hardness, hydrogen-form for demin), anion IX (chloride-form for nitrate/sulfate, hydroxide-form for demin). Resins: Purolite, Dow/DuPont Amberlite + Amberjet, Lanxess Lewatit, Rohm + Haas, Mitsubishi Diaion.

Specific applications:

Boron removal (selective) — Purolite S108 + Lewatit MK51, in SWRO permeate polishing.
Nitrate removal — strong-base anion (SBA) resin. Chloride-form regenerated with brine.
Arsenic removal — strong-base anion or specialty media (Bayoxide E33 iron oxide).
Uranium removal — strong-base anion.
Perchlorate removal — specialty bifunctional SBA (Calgon CalRes 2103, Purolite A532E).

3.3 Activated carbon adsorption

GAC (granular) in deep contactors. EBCT (empty-bed contact time) 5–30 min. PAC (powdered) dosed pre-treatment for transient taste/odor events.

Reactivation: thermal (high-temp 800–950 °C in low-O₂ atmosphere); restores ~85–90 % of virgin capacity. On-site reactivation furnaces — Calgon Carbon Corporation, Norit Americas, Cabot Norit.

3.4 Advanced Oxidation Processes (AOPs) — detail

UV/H₂O₂ — most common municipal AOP. UV photolyzes H₂O₂ to ·OH. Doses: 500–1000 mJ/cm² UV + 3–10 mg/L H₂O₂. Used at Water Replenishment District + Orange County GWRS (CA), Singapore PUB NEWater.
UV/O₃ — combined process; potentially synergistic; less common than UV/H₂O₂.
O₃/H₂O₂ (Peroxone) — common for taste/odor + 1,4-dioxane in pre-existing ozone plants.
Fenton (Fe²⁺ + H₂O₂) — pH ~ 3; industrial wastewater. Photo-Fenton + Electro-Fenton variants.

3.5 Emerging contaminants

PFAS (per- + polyfluoroalkyl substances) — long-chain (PFOA, PFOS, GenX) regulated by EPA NPDWR April 2024 with MCLs 4 ng/L PFOA + 4 ng/L PFOS + 10 ng/L PFHxS + PFNA + GenX (HFPO-DA). Removal: GAC (good for long-chain), IX (specialty resins), reverse osmosis, foam fractionation.
1,4-dioxane — IARC 2B carcinogen. EPA Reference Dose 0.03 µg/L. Removed by AOP (UV/H₂O₂) — too volatile for activated carbon, too small for RO.
NDMA + nitrosamines — formed by chloramination of dimethylamine precursors. UV photolysis at 254 nm effective.
Hexavalent chromium (Cr-VI) — regulated separately in CA (10 ppb MCL Sep 2024). Treatment: SBA IX or strong reducing agent + ferrous-induced co-precipitation.
Manganese — secondary MCL 50 µg/L; aesthetic + neurological concerns 2024+. Pre-oxidation + filtration with greensand or pyrolusite.
Cyanotoxins — Microcystin-LR + cylindrospermopsin + anatoxin-a. Removal: PAC + ozone + chlorination (caution: chlorine converts microcystin to harmless products only at certain residuals).

3.6 Boron removal

SWRO permeate often retains 1–2 mg/L boron (rejection ~ 75 % typical). WHO guideline 2.4 mg/L; agricultural irrigation < 0.5 mg/L. Methods: high-pH 2nd-pass RO (boron exists as borate ion B(OH)₄⁻, well-rejected) or boron-selective IX.

4. Drinking-water regulation

4.1 US — Safe Drinking Water Act 1974 (amended 1986 + 1996)

Authorizes EPA to set National Primary Drinking Water Regulations (NPDWR; enforceable MCLs) + Secondary (SMCL; aesthetic).

Key NPDWRs:

Total Coliform Rule + Revised TCR (RTCR) Jan 2016.
Surface Water Treatment Rule (SWTR) 1989 + Enhanced SWTR (ESWTR) 1998 + LT1ESWTR 2002 + LT2ESWTR Jan 2006 — 99.9 % Giardia + 99.99 % virus inactivation, Cryptosporidium added in LT2.
Total Trihalomethanes Rule 1979 + Stage 1 D/DBP 1998 + Stage 2 D/DBP 2006 — TTHM + HAA5 + bromate + chlorite.
Arsenic Rule 2001 — MCL 10 µg/L (reduced from 50).
Lead and Copper Rule (LCR) 1991 + LCR Revisions (LCRR) Dec 2021 + LCR Improvements (LCRI) Oct 2024 — lead action level 10 µg/L (decreasing from 15), mandatory full lead service line replacement within 10 years (with extensions for high-prevalence systems), 100 % inventory by Oct 16 2024.
Radionuclides Rule — Ra-226 + Ra-228 (combined 5 pCi/L), Uranium (30 µg/L), Beta + photon emitters (4 mrem/yr).
Groundwater Rule 2006.
PFAS NPDWR Apr 2024 — see §3.5.

NDWAC (National Drinking Water Advisory Council) advises EPA on rule-making.

4.2 EU — Drinking Water Directive (DWD) 2020/2184

Recast Dec 2020; transposition deadline Jan 12 2023. New parametric values:

Lead 5 µg/L (from 10, with 15-yr phase-in).
PFAS — sum of 20 PFAS 100 ng/L + total PFAS 500 ng/L.
Microplastics + bisphenol A + new disinfection by-products under watch list.
Risk-based management from catchment to tap.

4.3 Other jurisdictions

WHO Guidelines for Drinking-Water Quality 4th edition 1st addendum 2017; widely adopted by developing-country regulators.
Health Canada Guidelines for Canadian Drinking Water Quality.
Australian Drinking Water Guidelines 6 (NHMRC 2011, last amended 2022).
Singapore PUB drinking-water standards — WHO-based.

5. Wastewater collection

5.1 Conventional gravity sewers

Slope sized for self-cleaning velocity (≥ 0.6 m/s at peak flow), typically 0.2–2 % slope. Materials: vitrified clay pipe (VCP, ASTM C700), concrete (RCP, ASTM C76), PVC (ASTM D3034 SDR-35 + F679), HDPE, fiberglass (FRP, ASTM D3262). Manhole spacing 300–500 ft (90–150 m).

Design: Manning’s equation v = (1/n) · R^(2/3) · S^(1/2). n ≈ 0.013 PVC, 0.013 concrete, 0.011 vitrified clay.

5.2 Force mains

Pressurized; required for lift pumping over elevation rises. Materials: ductile iron (DI, AWWA C151), PVC pressure (AWWA C900 DR-14/18), HDPE (AWWA C906 DR 9–17), bar-wrapped pre-stressed concrete cylinder (PCCP, AWWA C301), bonded steel (AWWA C200).

5.3 Pressure + vacuum sewers

Low-pressure pressure sewers — grinder pump at each lot pushes effluent through small-diameter (50–100 mm) HDPE forcemain. Used for steep / rocky / scattered communities. E/One Sewer (Environment One Corp).
Vacuum sewers — central vacuum station maintains 0.5–0.7 bar vacuum; valves at each home admit slugs into the system. AIRVAC + Roediger + Iseki Vacuum.
STEP — Septic Tank Effluent Pump — onsite tank captures solids; small-diameter pressurized 1.25–2 in line conveys liquid effluent. Common in lake-district + low-density US west + AU + EU communities.

6. Wastewater treatment — preliminary + primary

6.1 Bar screens + comminutors

Bar screens — coarse (50–150 mm openings) + fine (5–20 mm) bar racks remove rags + debris. Mechanical raking: Vulcan, Lakeside Equipment, Headworks Inc, Brackett Green, Schreiber. Step-screen + drum-screen variants.
Comminutors / Muffin Monsters (JWC Environmental) — grinders dispatch solids in-line; less common in modern designs (just clog downstream).
Fine screens — perforated plate < 6 mm; required for MBR feed.

6.2 Grit chambers

Remove sand + grit (specific gravity > 2.65) to protect pumps + clarifier sludge collectors. Types: aerated grit (Pista® Smith & Loveless), vortex (Eutek Headcell + TeaCup, Lakeside Aeroflo), horizontal-flow (longitudinal).

6.3 Flow measurement

Parshall flume — most common open-channel primary device. Sized 1 in to 50 ft throat width. Palmer-Bowlus flume, H-flume, V-notch + rectangular weir also common. Ultrasonic + radar level transmitters (Endress+Hauser Prosonic / Micropilot, KROHNE OPTIWAVE, Siemens SITRANS).

6.4 Primary clarifiers

Gravity sedimentation removes 50–70 % TSS + 25–40 % BOD. Surface loading 30–50 m³/m²·d (700–1200 gal/d/ft²). Detention 1.5–2.5 hr. Circular or rectangular. Skimming for FOG (fats + oils + grease).

Sludge collectors: rotary scrapers (Walker Process + Brentwood + Westech Engineering Tow-Bro + suction headers).

7. Secondary biological treatment

7.1 Activated sludge — conventional + variations

Suspended-growth biological treatment: aerobic + anoxic + anaerobic basins recycle MLSS (mixed liquor suspended solids, 2000–5000 mg/L typical) cultivated to oxidize BOD + nitrify NH₄ + denitrify NO₃ + bio-P uptake.

Key parameters:

SRT (sludge retention time) = MLSS · V / WAS produced. Typical 4–10 d for BOD only, 8–25 d for nitrification.
HRT (hydraulic retention time) = V/Q. 4–8 hr typical.
F/M ratio = BOD load / MLSS mass = 0.2–0.5 d⁻¹ conventional.
DO 2 mg/L aerobic, < 0.5 anoxic, ~0 anaerobic.

Process variants:

Plug-flow aeration — classic; longest variant of CAS.
Step-feed — distributes load along basin.
Completely mixed — single CSTR.
Contact-stabilization — short contact + stabilization tank.
Oxidation ditch — looped-channel + brush aerators / surface aerators (Carrousel, Pasveer, Orbal).
SBR (Sequencing Batch Reactor) — single tank cycles fill / react / settle / decant. Aqua-Aerobic AquaSBR + Sanitaire ICEAS + Xylem Sanitaire.

7.2 Biological nutrient removal (BNR)

Configurations remove nitrogen + phosphorus:

A²O (anaerobic-anoxic-oxic) — anaerobic for bio-P release, anoxic for denitrification, oxic for nitrification + bio-P uptake.
UCT (University of Cape Town) — adds anoxic return-flow zone to protect anaerobic from nitrate.
MUCT (Modified UCT) — split anoxic.
Bardenpho 4-stage + 5-stage — extends N removal + anaerobic P uptake.
JHB (Johannesburg) — variant of UCT.
VIP (Virginia Initiative Plant) — variant of A²O / UCT.

7.3 Membrane bioreactors (MBR)

Activated sludge + membrane separation (UF or MF) replaces secondary clarifier. Achieves higher MLSS (8000–15000 mg/L) → smaller footprint, very high effluent quality (turbidity < 0.2 NTU, virtually no TSS), pathogen barrier.

Manufacturers + modules:

Suez (now Veolia Water Technologies) ZeeWeed 500D + 700B + 1500 — submerged hollow-fiber PVDF.
Kubota SmartMBR + RW + EW — submerged flat-sheet.
Pentair X-Flow Helix + Compact 33 + Aquaflex + Crossflow — sidestream + submerged.
Asahi Kasei Microza UNA-620A + MUNC.
Toray Hollow Fibre HSU-2520 + HCRJ + MBR HSL-2510 + HSU-1515.
Mitsubishi Chemical Aqua SX + SUR flat-sheet.
Memstar (Singapore) + Origin Water (China).

Major MBRs:

Ulu Pandan WRP (Singapore) ~ 320 MLD MBR-NEWater feed.
Brightwater (King County Seattle WA) 200 MLD.
Henriksdal (Stockholm) 864 MLD (world’s largest MBR; Veolia 2020+).
Beixiaohe Beijing + Wenyu River Beijing + Tampa Bay Reservoir / HCWRF + Stickney Chicago MWRD (large North American).

7.4 Moving Bed Biofilm Reactor (MBBR) + IFAS

Plastic carriers (typically polyethylene, 5–30 mm chips with high surface area 500–1200 m²/m³) provide biofilm attachment surface. Carriers float freely; retention screens at exit.

Carriers: AnoxKaldnes K1 + K3 + K5 + Z-MBBR carriers (Veolia), Headworks BIO ActiveCell, Entex Technologies BioPortz, Biowater + Hydroxyl Systems Bio2 Sphere.

IFAS (Integrated Fixed-Film Activated Sludge) — combines MBBR carriers in an activated-sludge basin, retrofitting capacity into existing plants.

7.5 Trickling filters + RBCs

Trickling filter — wastewater spray over plastic random-packing media (typical Brentwood Vinyl Core CF-1900, Munters CF-1900) or rock (legacy). Aerobic biofilm.
Rotating Biological Contactor (RBC) — discs partially submerged + rotated, biofilm on disks. Walker Process + USFilter Surfact-Aire.
Used less in new construction but still common in retrofits + small plants.

7.6 Anaerobic treatment

UASB (Upflow Anaerobic Sludge Blanket) reactors for high-strength industrial wastes (food processing, distillery, paper). Methane generation. Veolia Biothane UASB, Paques Biopaq, Voith UASB.

8. Tertiary treatment

8.1 Tertiary filtration

After secondary clarification, further reduce TSS + turbidity. Sand filter (dual-media), cloth-disk filters (Aqua-Aerobic AquaDisk + AquaDrum, Mecana, Aqua-Aerobic), MicroScreen (Schreiber + Salsnes), membrane MF/UF.

8.2 Nutrient polishing

Denitrifying filter — methanol-fed deep-bed sand; biological denitrification.
Phosphorus polishing — alum or ferric chloride dose pre-tertiary filter to precipitate residual P. Targets 0.05–0.5 mg/L TP.
Reactive media — adsorptive media (e.g., Phoslock, Bayoxide E33-PAS).

8.3 Final disinfection

Chlorination + dechlorination (SO₂ + Na bisulfite or thiosulfate) — historical standard.
UV disinfection — increasingly preferred; no DBP. Trojan UV3000Plus + UV4000Plus + UV Signa + ECT, Wedeco TAK 55 + Spektron, Xylem Wedeco K + Aqua Net.
Ozonation — also for taste/odor; some plants (Tampa Bay) ozonate to support potable reuse.
Peracetic acid (PAA) — wet-weather + low-DBP alternative; Solvay + Evonik + PeroxyChem.

9. Biosolids management

9.1 503 Rule (40 CFR 503)

EPA 1993 Part 503 biosolids regulations classify:

Class A (pathogen-free; can be sold/given to public, e.g., compost): fecal coliform < 1000 MPN/g; vector attraction reduced.
Class B (pathogen-reduced; land-application with site restrictions): fecal coliform < 2×10⁶ MPN/g.
EQ (Exceptional Quality) — meets Class A + pollutant ceilings.

Pollutant limits (Table 3): As 41, Cd 39, Cu 1500, Pb 300, Hg 17, Ni 420, Se 100, Zn 2800 (all mg/kg dry).

9.2 Anaerobic digestion

Mesophilic (35 °C) or thermophilic (55 °C). Detention 15–30 d mesophilic. Methane content of biogas ~ 60 %, CO₂ ~ 40 %, with H₂S + siloxanes contaminants.

Designs: complete-mix (single + dual stage), plug-flow, EGSB Expanded Granular Sludge Bed.

Equipment: Anaergia OmnivoreTM, Veolia Bioactiflo, Suez Sludge Cleansys, Cambi Thermal Hydrolysis (THP) pre-treatment for high-solids high-rate digestion.

9.3 Biogas utilization (CHP)

Combined heat + power engines: Caterpillar CAT G3500 + G3600, Jenbacher (INNIO J series), Wärtsilä 31SG, Cummins Q series. ICE drives generator; jacket-cooling heat recovers to digester. Typical 30–35 % electrical + 40–50 % thermal efficiency.

Biogas upgrading to renewable natural gas (RNG): membrane (Air Liquide MEDAL + Evonik SEPURAN Green + Pentair Haffmans), PSA (pressure swing adsorption), water scrubbing, amine scrubbing.

9.4 Thermal drying + incineration

Direct-fired drum dryers (Andritz, Komline-Sanderson, FENIX), belt dryers (HUBER BT, Veolia BIOCON, Andritz BDS), fluidized-bed incineration (FB; Andritz, Walker Process, Veolia Pyrofluid). Pyrolysis + gasification + hydrothermal liquefaction (HTL) emerging.

9.5 Land application + composting

Class B biosolids land-applied at agronomic rates per site restrictions. Composting (Class A) via windrow, aerated static pile, in-vessel (Compostex + Engineered Compost Systems).

9.6 Alkaline stabilization

Quicklime addition raises pH > 12 + temperature briefly; kills pathogens. N-Viro Soil, RDP Technologies + ChemScan Lystek THP, Schwing Bioset.

10. Wastewater regulation

10.1 Clean Water Act 1972

Authorizes NPDES (National Pollutant Discharge Elimination System) permits for any point-source discharge. Permits set effluent limits based on:

Technology-based — BAT/BCT/BPT for industry; secondary treatment standards 40 CFR 133 for POTWs (30 mg/L BOD + 30 mg/L TSS + pH 6–9).
Water-quality-based — case-by-case for receiving water 303(d) impairments; based on TMDL (Total Maximum Daily Load) allocations + WLAs (waste-load allocations).

303(d) lists + TMDLs drive nutrient + sediment + PCB + Hg + bacteria reductions across major basins (Chesapeake Bay TMDL 2010, Mississippi River + Gulf hypoxia, Long Island Sound, Puget Sound).

10.2 EU Urban Wastewater Treatment Directive (UWWTD)

Original 91/271/EEC + recast adopted 2024 — implementation by 2035–2045. Stricter nutrient removal, full coverage of agglomerations > 1000 PE, mandatory micropollutant treatment (quaternary treatment) for plants > 150,000 PE by 2045, extended producer responsibility for pharmaceutical + cosmetic industries.

10.3 Pretreatment + industrial discharges

POTWs implement local pretreatment programs (40 CFR 403) — categorical industrial users must comply with EPA national categorical standards (40 CFR 405–471). Specifically: metals 433.16, petroleum refining 419, semiconductor + electronics 469.

11. Water reuse

11.1 Reuse categories + standards

Non-potable urban: irrigation, fire protection, toilet flushing. WateReuse Title 22 California (1978; revised 2018) — disinfected tertiary recycled water with turbidity ≤ 2 NTU + coliform < 2.2 MPN/100 mL.
Agricultural: food crops eaten raw (Cal Title 22), processed (Cal Title 22 lower tier), non-food.
Industrial process: cooling towers, boiler feedwater (varies by use).
Environmental restoration: wetland creation, stream augmentation.
Indirect Potable Reuse (IPR): reclaimed water → environmental buffer (groundwater recharge, surface reservoir) → drinking-water treatment plant. Mandatory environmental retention time varies (e.g., California 6 months).
Direct Potable Reuse (DPR): reclaimed water → drinking water treatment plant directly (no environmental buffer). California adopted DPR regulations Dec 2023 (effective Oct 2024).

11.2 Major reuse projects

NEWater Singapore (2003+) — 5 plants providing ~ 40 % of national demand. Process: MBR → RO → UV. PUB Singapore + KeppelSeghers + Hyflux + Sembcorp.
Orange County Water District Groundwater Replenishment System (GWRS) — Fountain Valley CA, ~ 100 MGD (380 MLD) since 2008, expanded to 130 MGD (500 MLD) in 2023. Process: MF → RO → UV/H₂O₂ AOP. World’s largest IPR plant. Recharges Orange County groundwater basin.
West Basin Edward C. Little Water Recycling Facility El Segundo CA — 40+ MGD, multiple uses (industrial cooling at Chevron refinery, irrigation, barrier injection).
Big Spring Colorado River Municipal Water District TX — 2 MGD DPR since 2013; first US DPR plant.
Wichita Falls TX — Cloud Park DPR 2014 (drought emergency).
Cape Town, Goreangab Reclamation Plant Windhoek Namibia — DPR since 1968 (oldest).
Hyderabad PSA Recycling Plant, Tianjin Beidagang (China), Kraków + Brussels SOLIDIA.
California Pure Water San Diego + Pure Water Antelope Valley + Replenishment District + Las Virgenes-Triunfo Pure Water Project under construction or commissioning 2025–2030.

11.3 PURIFICS Pathwater + Hi-PuriT + Mirzania De Nora EDS Membrane Distillation — emerging tertiary technologies

12. Desalination (detail)

12.1 Thermal processes

MSF (Multi-Stage Flash) — historical workhorse; 18–28 stages; brine flashed at progressively lower pressure. Specific energy ~ 12–15 kWh_thermal + 4 kWh_electric per m³. Dominant Middle East large plants pre-2010.
MED (Multi-Effect Distillation) — horizontal-tube falling-film evaporators; lower temp than MSF; specific energy 6–8 kWh_thermal + 1.5 kWh_electric per m³. IDE + SIDEM + Doosan. Often paired with thermal vapor compression (TVC) or mechanical vapor compression (MVC).
MED-TVC + MSF-Brine recirculation — hybrid combined with co-generation gas turbine.

12.2 SWRO — Seawater Reverse Osmosis

Energy: ~ 3.0–3.5 kWh/m³ specific energy (SEC) including pretreatment + post-treatment + intake/outfall — close to thermodynamic minimum ~ 1.06 kWh/m³ at 50 % recovery.

Pretreatment is critical (membranes intolerant of fouling): coagulation + DAF/sedimentation + multi-media or UF filtration + cartridge filter + acid (ATA antiscalant — Avista Vitec, Genesys LF, Nalco PermaCare).

12.3 Energy recovery devices (ERDs)

Critical for energy efficiency — pressurized brine reject (60–70 bar) drives ERD to pressurize incoming feed.

Pressure exchangers (isobaric) — rotary ceramic disk transfers pressure directly with > 96 % efficiency. Energy Recovery Inc PX-Q300 + PX-Q400 (San Leandro CA; SWRO market dominant). Flowserve DWEER.
Pelton wheel turbine — older technology; ~ 80 % efficient; still used in smaller plants. Bahnson, Andritz Hydro.
Hydraulic turbocharger — Energy Recovery Inc ERI Turbocharger.

ERD adoption brought SEC from > 6 kWh/m³ (1990s) to 3 kWh/m³ (2020s).

12.4 Pretreatment options

Open intake — surface seawater intake; high biofouling + variable quality + jellyfish events (Hadera Israel 2013 jellyfish bloom, Saudi Arabian Gulf events).
Sub-sea / sub-seabed intake — beach wells + horizontal directional drilled (HDD) intakes filter seawater through aquifer. Far cleaner; lower pretreatment; higher CAPEX.
Pretreatment trains — coagulation + DAF/sedimentation + dual-media filter → cartridge filter; or coagulation + UF → cartridge filter (UF pretreatment increasingly preferred for highly fouling waters).

12.5 Brine management

Desalination brine ~ 55,000–70,000 mg/L TDS + concentrated antiscalant + decoagulant residual. Disposal: ocean outfall with diffusers (Carlsbad uses CW co-discharge with NRG Encina power plant), evaporation ponds (inland), zero-liquid discharge ZLD via brine-concentrator + crystallizer (Aquatech + GE Water + Veolia HPD + Saltworks).

12.6 Hybrid + emerging desal

Forward osmosis (FO) — Modern Water + Trevi Systems + Oasys Water; draw solution + low-temp regen. Pilot only.
Membrane distillation (MD) — Memsys + Aquastill + Solar Spring; thermal-membrane hybrid. Pilot only.
Capacitive deionization (CDI) + flow-electrode CDI — low-TDS brackish only.

13. Specific industrial considerations

13.1 Power-plant water

Cooling water: once-through (banned in CA + NY for once-through coastal intakes by 2030), cooling tower makeup (3–5 cycles of concentration), dry/hybrid cooling.
Boiler feedwater: high-purity demineralized (RO + mixed-bed IX or RO + EDI electrodeionization). Supercritical boilers require < 1 ppb total dissolved solids.
FGD (flue gas desulfurization) wastewater: ELG (Effluent Limitations Guidelines) rules — selenium, mercury, arsenic, nitrate limits. ZLD trend.

13.2 Semiconductor + microelectronics

Ultra-pure water (UPW): TDS < 0.1 µg/L, TOC < 1 µg/L, particles < 0.05 µm, bacteria 0–1 CFU/L. Multi-pass RO + UV + EDI + UF polish loop. Plants: Intel + TSMC + Samsung + Micron + Global Foundries fabs (10–30 MGD UPW per major fab).

13.3 Food + beverage

Dairy + brewery + soft drink + meat processing. High organic loading (BOD 1500–10,000 mg/L). UASB + DAF + MBR + ozone polish. Major OEMs: Veolia, Suez, GEA, Alfa Laval, Tetra Pak Plant solutions.

14. Software + modeling

EPANET 2.2 (USEPA, Lewis Rossman) — water distribution hydraulic + water quality simulation. Free.
WaterCAD + WaterGEMS (Bentley Systems) — commercial GIS-integrated.
InfoWorks WS Pro + ICM (Innovyze, now Autodesk) — water + wastewater network modeling.
PCSWMM + SWMM5 (CHI + USEPA) — stormwater + wastewater.
GPS-X (Hydromantis Inc) — wastewater process simulation; activated sludge + BNR + MBR.
BioWin (EnviroSim Associates) — wastewater process simulation; ASM Activated Sludge Models 1–3.
WEST (DHI; Mike WEST) — water + wastewater process.
SUMO (Dynamita) — wastewater process modeling.
AQUASIM (EAWAG) — research-grade process kinetics.
MIKE FLOOD + MIKE URBAN + MIKE+ (DHI) — integrated urban water cycle.

15. Asset management + intelligent water

SCADA — Schneider Electric ClearSCADA + EcoStruxure, Rockwell FactoryTalk, Siemens SIMATIC PCS 7, Inductive Automation Ignition, AVEVA InTouch + System Platform.
Smart metering — Sensus iPERL + RG3 + iCON, Itron Galaxy + Riva, Kamstrup Flow IQ + Multical 21 + Multical 62, Diehl Hydrus + Aquarius + Altair, Badger ORION + M2000.
Hydraulic transient analysis — Bentley HAMMER, KYPipe Surge, Dyatel SystemTransient, AFT Impulse.
Asset GIS — ESRI ArcGIS Utility Network + Esri Local Government Solutions.
Leakage detection — acoustic correlation (Echologics, Vivax Metrotech, Heath Consultants), district metered areas (DMAs), pressure management.

Compendium

Explorer

Water + Wastewater Treatment — Sources, Unit Processes, Regulations, Reuse, Desal