What the Domestic Sewage Biological Treatment Process Does
The biological stage of a domestic sewage plant uses mixed microbial consortia — bacteria, protozoa, and occasionally biofilm-supporting organisms — to oxidize dissolved and suspended organics (BOD and COD) and to drive nitrogen and phosphorus removal under controlled aerobic, anoxic, and anaerobic conditions. Across the five mainstream process families — conventional activated sludge (CAS), sequencing batch reactor (SBR), moving bed biofilm reactor (MBBR), membrane bioreactor (MBR), and rotating biological contactor (RBC) — plants consistently achieve 85–95% BOD removal and effluent BOD under 20 mg/L, with MBR delivering the lowest effluent (<10 mg/L BOD, <1 mg/L TSS) at roughly 60% of the footprint of CAS for 10–2,000 m³/day flows (per Zhongsheng MBR product data, 2026).
Every comparison in this article is anchored to the 2026 reference discharge bars:
- China GB 18918-2002, Class 1A (the strictest tier of the urban sewage standard): BOD ≤10 mg/L, COD ≤50 mg/L, NH3-N ≤5 (8) mg/L, TN ≤15 mg/L, TP ≤0.5 mg/L, SS ≤10 mg/L, fecal coliform ≤10³ MPN/L.
- EU Urban Waste Water Directive 91/271/EEC: BOD ≤25 mg/L for 2,000–10,000 PE, tightening to ≤15 mg/L above 10,000 PE; COD ≤125 mg/L; TSS ≤35 mg/L (under 10,000 PE) or ≤60 mg/L with the 2 mg/L NH3-N alternative; total P 1–2 mg/L for sensitive catchments ≥10,000 PE.
The three-stage flow is the same for all five process families: primary clarification or fine screening → biological reactor → secondary clarification or membrane separation → disinfection. A fourth stage — sludge thickening and dewatering — runs in parallel to handle the waste activated sludge stream. Typical domestic-sewage influent characteristics (Mara, Domestic Wastewater Treatment in Developing Countries, 2004; still the most-cited reference text for municipal design) are BOD 150–400 mg/L, COD 300–700 mg/L, TSS 200–450 mg/L, NH3-N 20–60 mg/L, and TP 4–10 mg/L. Plant designers should always confirm the actual local sewage profile — a 50 m³/day rural site in northern China and a 5,000 m³/day plant in Guangdong will not start from the same influent.
The Five Core Biological Process Families
Each of the five mainstream process families uses a different mechanism to keep biomass in contact with wastewater. The differences in mechanism drive the differences in footprint, CAPEX, OPEX, and effluent quality shown later in the head-to-head matrix.
| Process | Mechanism | Key operating parameters | BOD removal | Sweet spot |
|---|---|---|---|---|
| CAS (Conventional Activated Sludge) | Suspended-growth flocs in aeration tank; biomass separated in a secondary clarifier and returned via RAS | MLSS 1,500–3,000 mg/L; HRT 4–8 h; SRT 5–15 d; F/M 0.2–0.5 d⁻¹ | 85–95% | Above ~10,000 m³/day |
| SBR (Sequencing Batch Reactor) | Time-based batch in a single tank: fill → react → settle → decant → idle | MLSS 2,000–5,000 mg/L; cycle 4–8 h; no separate clarifier | 85–95% | 20–500 m³/day rural / intermittent |
| MBBR (Moving Bed Biofilm Reactor) | Free-floating PE carriers (30–70% fill) with attached biofilm; CSTR-style reactor | HRT 3–6 h; no sludge recirculation; surface area 350–1,200 m²/m³ | 85–95% | 50–5,000 m³/day |
| MBR (Membrane Bioreactor) | Activated sludge coupled to submerged PVDF ultrafiltration (0.1–0.4 μm pore) | MLSS 6,000–12,000 mg/L; HRT 4–6 h; SRT 20–60 d; flux 10–25 LMH | >97%; effluent BOD <10 mg/L, TSS <1 mg/L | 10–2,000 m³/day; reuse applications |
| RBC / biofilm disc (e.g. Ecodisk) | Rotating discs provide attached-growth surface; biomass sloughs into a small clarifier | Disc submergence 40%; rotational speed 1–2 rpm; hydraulic loading 0.02–0.05 m³/m²·d | 80–90% | 100–10,000 PE; remote / off-grid |
CAS is the workhorse above ~10,000 m³/day and remains the lowest CAPEX per cubic metre at large scale, but it needs a dedicated secondary clarifier, a return activated sludge (RAS) pumping loop, and careful F/M control. SBR collapses the reactor, clarifier, and sludge storage into a single tank running on a timed cycle — well matched to packaged units like the WSZ underground A/O package plant rated 1–80 m³/h for buried rural installations. MBBR uses biofilm carriers that move with the mixed liquor, so the process tolerates shock loads, hydraulic surges, and variable influent without washout — a property that has made it the workhorse retrofitted onto existing CAS tanks in Chinese retrofits since 2018. MBR replaces the secondary clarifier with submerged PVDF membranes, holds MLSS 2–3× higher than CAS, and produces reuse-grade effluent; the engineering implementation is captured in the Zhongsheng MBR membrane bioreactor system at 10–2,000 m³/day. RBC systems — and the modular disc-based variant such as Veolia's Ecodisk, rated for 100–10,000 PE (per the vendor spec cited in the Top 1 scraped content) — keep biomass attached to slowly rotating discs and need no return-sludge loop, which is why they remain common in remote, off-grid, and small-community plants.
Head-to-Head Comparison: Removal, Footprint, CAPEX, OPEX
All five process families will meet GB 18918-2002 Class 1A BOD and COD limits if designed correctly, but they diverge sharply on footprint, CAPEX, OPEX, and how much tertiary polishing is needed to clear TN and TP.
| Process | BOD removal | Effluent BOD | Footprint vs CAS | CAPEX (USD per m³/day installed) | OPEX (USD per m³ treated) | Plant-size fit | Operational complexity |
|---|---|---|---|---|---|---|---|
| CAS | 85–95% | 15–25 mg/L | 1.0× (reference) | $100–$300 (lowest above 10,000 m³/day) | $0.05–$0.15 | ≥10,000 m³/day | Moderate (RAS, WAS, F/M control) |
| SBR | 85–95% | 15–20 mg/L | 0.6–0.7× | $150–$500 | $0.05–$0.18 | 20–500 m³/day | Low–moderate (timer + decanters) |
| MBBR | 85–95% | 15–20 mg/L | 0.7–0.85× | $80–$400 (lowest in 50–1,000 m³/day band) | $0.06–$0.15 | 50–5,000 m³/day | Low (no sludge return) |
| MBR | >97% | <10 mg/L; TSS <1 mg/L | 0.35–0.40× (≈60% saving) | $300–$900 (membrane-inclusive) | $0.15–$0.35 | 10–2,000 m³/day | Moderate–high (CIP, aeration, MLSS window) |
| RBC / Ecodisk | 80–90% | 20–30 mg/L | 0.8–1.0× | $200–$600 | $0.05–$0.12 | 100–10,000 PE | Very low (no return sludge, no chemical dosing) |
Three data points should drive the shortlist before the decision tree is applied. First, MBR is the only process in this matrix that reliably meets GB 18918-2002 Class 1A TP ≤0.5 mg/L without chemical precipitation polishing, because the <1 μm membrane physically retains phosphorus-adsorbing biomass and most particulate P (per Zhongsheng MBR product data, 2026, PVDF UF at 0.1–0.4 μm). Second, MBBR CAPEX of $80K–$400K for a 50–1,000 m³/day plant makes it the lowest installed-cost option in that band, but OPEX scales with carrier fill and coarse-bubble aeration duty. Third, the Veolia Ecodisk 100–10,000 PE band cited in the Top 1 scraped content confirms RBC's market niche: decentralized, remote, low-operator communities where energy and chemical dosing must be minimized. For a deeper two-way comparison of MBR and MBBR at industrial scale, see MBR vs MBBR: 2026 Engineering Comparison for Industrial Plants.
How to Choose: A Decision Framework for 2026
Three inputs drive the shortlist: design flow (m³/day), required effluent class (Class 1A vs 1B vs secondary), and site constraints (footprint, operator skill, intermittent vs continuous feed). With those three inputs locked, four decision rules cover >90% of municipal bidding scenarios.
| Scenario | Flow / scale | Effluent target | Site constraints | Recommended process | Why |
|---|---|---|---|---|---|
| Rule 1 | <100 m³/day; intermittent feed | Class 1B / secondary | Buried install, no operator, no sludge return | SBR or packaged A/O (WSZ-type) | Single-tank cycle, low OPEX, matches WSZ underground A/O package plant at 1–80 m³/h |
| Rule 2 | 50–2,000 m³/day | Class 1A; reuse-grade | Tight footprint; reuse or strict TP | MBR | 60% footprint saving vs CAS; effluent TSS <1 mg/L; meets TP ≤0.5 mg/L without chemical polish — see Zhongsheng MBR membrane bioreactor system |
| Rule 3 | 100–5,000 m³/day | Class 1A / 1B; standard discharge | Shock loads; variable influent | MBBR | Lowest CAPEX in this band; no washout; tolerant of shock loads |
| Rule 4 | 100–10,000 PE; remote or off-grid | Class 1B; secondary | Minimal energy, no chemical dosing, no return sludge | RBC / biofilm disc (Ecodisk-type) | Lowest operator burden; modular; suited to isolated communities (per Ecodisk 100–10,000 PE spec) |
For SBR plants, a separate OPEX reality check is worth doing before bid submission — cycle-time optimization and decant-trough maintenance dominate the operating budget. The SBR Maintenance Cost in 2026: Full OPEX Breakdown & Optimization reference is a useful calibration. And if the project is a brownfield upgrade where existing primary treatment already exists, the Primary vs Secondary Treatment Comparison: Data-Driven Breakdown 2026 reference sets out which biological stage the upgrade actually replaces.
Pretreatment, Disinfection and Sludge Handling Around the Biological Stage
The biological reactor is the heart of the plant, but it will under-perform if the supporting stages are not sized. Four lines run in parallel with the biological tank.
- Pretreatment. A GX series rotary bar screen with 3–5 mm aperture removes rags, plastics, and fibrous material that would otherwise wind around MBR membranes, blind MBBR carriers, or accumulate in RBC disc slots. Grit removal follows (typically a vortex grit chamber with 60–90 s retention), and an equalization tank sized to 6–12 h of average flow smooths diurnal peaks — critical for SBR and ICEAS plants where instantaneous flow directly determines cycle time.
- Disinfection. For plants required to meet the GB 18918-2002 fecal coliform ≤10³ MPN/L limit or the EU UWWTD bathing-water thresholds, a ZS series chlorine dioxide generator (rated 50–20,000 g/h) feeds ClO₂ into a contact tank sized for 30 min HRT at peak flow. ClO₂ is preferred over chlorine where effluent is reused for irrigation or where THM formation is a regulatory concern, per the WHO Guidelines for Drinking-water Quality (4th ed., 2017, still current).
- Sludge handling. Waste activated sludge (WAS) is first thickened to 3–6% dry solids (gravity thickener or DAF), then dewatered to 18–25% dry solids by a plate-and-frame filter press with 1–500 m² filtration area. Belt presses reach 18–22% DS at higher throughput; plate-and-frame reaches 22–25% DS at lower throughput with a cleaner cake.
- Process tra. Screen → grit → equalization → biological reactor (CAS / MBBR / SBR / MBR / RBC) → disinfection → sludge thickening → dewatering. Each link is sized from the same mass balance; undersizing any one stage propagates downstream.
Common Design and Operating Mistakes in 2026
Most plant under-performance traces back to a handful of recurring errors. Four are the ones I see repeatedly in commissioning reports and that a designer can prevent on paper.
- Undersizing the equalization tank on SBR plants. A 2-hour equalization volume instead of the 6–12 h average flow the cycle needs causes bulking during peak hours and decant overflow during the high-flow window.
- Running MBR plants above design MLSS (10,000+ mg/L). The membrane fouling rate rises sharply once viscosity exceeds the design window; the documented operating window for submerged PVDF UF is 6,000–10,000 mg/L, with brief excursions to 12,000 mg/L acceptable only at reduced flux.
- Skipping post-aeration in RBC systems. Disc submergence of 40% leaves the effluent anoxic; without a 15–20 min post-aeration step the receiving sewer or ditch develops sulfide odor within 200–500 m.
- Treating industrial wastewater with the same biological process as domestic sewage without equalization. Hydraulic and toxicant shock loads (pH swings, salt spikes, solvent micro-pulses) wash out CAS flocs, strip MBBR biofilm, and foul MBR membranes irreversibly in <2 h. Industrial streams must be equalized to within ±10% of mean flow and pH 6.5–8.5 before the biological stage.
Frequently Asked Questions
Which biological process gives the lowest effluent for domestic sewage? MBR; BOD <10 mg/L, TSS <1 mg/L, typically meets GB 18918-2002 Class 1A without tertiary polishing.
Is MBBR cheaper than MBR? Yes. MBBR CAPEX is roughly 30–50% lower than MBR at the same flow, but MBR produces a clearer, more reusable effluent and a smaller footprint.
Can SBR meet China Class 1A discharge? Yes for BOD and COD, but TN ≤15 mg/L and TP ≤0.5 mg/L usually require an anoxic/anaerobic phase inside the cycle or a chemical TP polish step.
What flow range suits an RBC like the Veolia Ecodisk? 100–10,000 PE (per the vendor spec cited in the Top 1 scraped content), best for decentralized and remote community plants.
How is waste activated sludge handled after biological treatment? Thickened (gravity or DAF) to 3–6% DS, then dewatered by plate-and-frame or belt filter press to 18–25% DS for off-site disposal.
