Assumptions (used everywhere)
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Plant flow (average): Q = 500 m³/day = 20.833 m³/hr (continuous).
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Representative influent (from your earlier inputs / typical slaughterhouse): COD 5,000 mg/L, BOD₅ = 2,000 mg/L, TSS = 2,000 mg/L, FOG = 1,000 mg/L.
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Anaerobic (UASB) COD removal: 65% (design basis). Result used downstream.
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Design safety / sizing factors: apply 20–30% spare capacity or safety factor on hydraulic areas/ membrane areas and 30% on membrane area unless noted. Where design ranges exist I choose conservative mid/high values for reliability. Design references are shown inline.
1) Coarse screening
Purpose: remove rags, large bones, plastics; protect downstream pumps/ drum screen.
Design:
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Bar spacing: 10 mm (coarse) for slaughterhouse (use a finer 6 mm if many small solids).
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Peak/continuous flow: 21 m³/hr → choose screening capacity for 30 m³/hr to allow surges.
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Typical item: coarse channel screen or perforated step screen. Provide a wash/compactor or manual basket.
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Headloss: design ~10–50 mm. Provide trash bin sized for daily accumulation (estimate ~10–20 kg/day coarse solids; depends on operations).
Recommendation: stainless steel (SS304/316) with a bypass manual screen and access for cleaning.
2) Oil trap / grease interceptor (before equalization)
Purpose: remove free-floating oils/grease to reduce downstream fouling.
Design basis & calculation:
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Use 2 hours hydraulic detention (typical interceptor detention 30 min–2 hr for food industry grease traps; choose 2 hr to be conservative).
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Volume = Q_hr × detention = 20.833 m³/hr × 2 hr = 41.7 m³.
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Use two compartments (inlet baffle + separation zone + cleanout sump). Depth ~1.2–1.8 m. For compact footprint pick 1.5 m depth → plan area = 41.7 / 1.5 = 27.8 m² → e.g., 7 m × 4 m footprint.
Notes: provide skimming port, access manholes, sludge/grease collection tray. Material: GRP or SS depending on budget.
3) Equalization tank (with drum solid separator)
Purpose: dampen flow/load variations; allow coarse solids separation (drum screen) and chemical dosing if needed.
Design assumptions:
Calculation:
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Volume = Q × 1.5 day = 500 m³/day × 1.5 = 750 m³.
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Provide mixing (slow mixers) to avoid short-circuiting; mechanical mixers or coarse bubble aeration to prevent septic conditions. If anaerobic pre-treatment is used immediately after EQ, DO control may be needed.
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Drum screen: install as bypass/inline solids removal inside EQ: design for continuous throughput ~21 m³/hr. Typical drum screen flux (vendor dependent) handles dozens to hundreds m³/hr per unit; specify vendor model for Q=25 m³/hr with 1–2 mm perforations. Provide rakes, wash water and 1.5–2 hour solids hopper.
Recommendation: EQ as rectangular concrete tank in 2 compartments (settling & buffer) with level control for equalization and automatic dosing skid for coagulant / pH if needed.
4) DAF (FOG recovery)
Purpose: remove dissolved/ emulsified oils & greases, remaining floatables; concentrate FOG for rendering or digestion.
Design criteria (industry ranges):
Calculation:
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Hourly flow Qh = 20.833 m³/hr.
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Required DAF area A = Qh / HLR = 20.833 / 6 = 3.472 m². Apply practical plate-pack geometry (effective area multiplies by plate factor 3–6). Choose a plate-pack DAF with plate pack factor 5 → actual tank footprint ≈ 3.472 / 5 = 0.695 m² (very compact) — vendors typically give modular units; choose a standard DAF rated for 25 m³/hr.
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Typical recycle (saturation) ratio 10–20% → design recirc pump capacity ~ 5 m³/hr and air saturator at 4–6 barg. Sludge (float) concentration: 3–6% solids; expected daily DAF float ~ 200–350 kg dry solids (depends on FOG/TSS removal %).
Notes: include polymer/coagulant dosing skid upstream, skimmer and float hopper with pump to sludge tank. Material: SS316 recommended.
5) Primary clarifier (after DAF)
Purpose: remove remaining settleable solids before biological treatment.
Design criteria:
Calculation:
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Flow Q = 500 m³/day.
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Area A = Q / SOR = 500 / 30 = 16.67 m². Add 30% safety factor → ~21.7 m². Choose a circular clarifier: area πD²/4 = 21.7 → D = sqrt(4×21.7/π) = ≈5.26 m → choose 6 m diameter clarifier.
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Depth: 3.0–3.5 m. Volume ≈ area × depth ≈ 21.7 × 3.5 ≈ 76 m³. Provide sludge hopper and scum removal.
Notes: Provide inlet baffle, flow-distribution box, sludge withdrawal pump to sludge handling.
6) Anaerobic reactor — UASB
Purpose: remove bulk COD, generate biogas for energy recovery.
Design basis used earlier (restated with steps):
Calculation:
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Required UASB volume = COD load / OLR = 2,500 / 4 = 625 m³.
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HRT = Volume / Q = 625 / 500 = 1.25 day = 30 hours. (UASB HRTs typically 12–48 hr for strong wastes; 30 h is conservative.)
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COD removal expected ≈ 60–70% → design 65% (1,625 kg/day removed). Remainder COD to aerobic stage ≈ 875 kg/day → concentration post-UASB ~1,750 mg/L COD.
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Gas handling: CH₄ ≈ 0.35 m³ CH₄/kg COD removed → CH₄ ≈ 569 m³/day. Provide gas holder, condensate trap, H₂S mitigation (if H₂S high), flare/CHP skid.
Reactor geometry & internals: typical UASB height 4–6 m; provide gas–solid–liquid separator (GSL), influent distribution, sludge withdrawal. Material: concrete with internal lining or FRP/steel if modular.
7) Aerobic treatment — Aeration tank 1 → Clarifier 1 → Aeration tank 2 → Clarifier 2 → Disinfection
You specified two-stage aerobic with two clarifiers. I size the total aerobic system to reach polishing BOD ≈ <30 mg/L (ready for filtration + RO).
Design approach: compute required aerobic volume from remaining BOD load after UASB and volumetric loading (kg BOD/m³·day).
Step A — Estimate BOD load to aerobic:
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Influent BOD load = 500 × 2,000 mg/L = 1,000 kg BOD/day.
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Assume UASB removes 60% BOD (approx. aligned with COD removal), so BOD to aerobic = 1,000 × (1 − 0.60) = 400 kg/day. (This matches earlier quick calc.)
Step B — Aerobic volumetric loading (typical):
Calculation:
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Aeration basin total volume V = BOD to treat / VLR = 400 / 1.5 = 266.7 m³.
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Split into two identical aeration tanks: V1 = V2 = 133.3 m³.
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HRT total = V / Q = 266.7 / 500 = 0.533 day = 12.8 hr → per tank ≈ 6.4 hr HRT each (reasonable for high-rate activated sludge).
MLSS / SRT (guidance):
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Choose MLSS near 3,500–4,500 mg/L for strong industrial BOD; pick 4,000 mg/L.
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Calculate biomass mass: X × V = 4 kg/m³ × 266.7 m³ = 1,066.8 kg MLSS (total volatile solids basis).
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Required sludge wasting (to maintain SRT) depends on chosen SRT; pick SRT = 8–12 days for conventional AS (choose 10 days). Then waste sludge VSS/day ≈ biomass / SRT = 1,066.8 / 10 = 106.7 kg VSS/day (dewater accordingly).
Aeration (O₂) requirement:
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O₂ required for carbonaceous BOD removal ≈ 1.42 kg O₂ / kg BOD removed (standard). So O₂ = 400 × 1.42 = 568 kg O₂/day.
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Aeration energy estimate: Standard Aeration Efficiency (SAE) for fine-bubble diffused aeration around 2.5 kg O₂/kWh (practical). Electrical energy ≈ O₂ / SAE = 568 / 2.5 = 227 kWh/day → average power ≈ 9.5 kW. (This is an indicative figure; blower and diffuser selection will refine it.)
Clarifiers (secondary) sizing:
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Secondary clarifier SOR design basis: 30 m³/m²·day average (range 24–33). I’ll use 30 m³/m²·day. Include RAS flow in calculation (assume RAS = 100% of influent flow, i.e., equal to 500 m³/day). So flow to clarifier = influent (500) + RAS (500) = 1,000 m³/day.
Area per clarifier = (flow to be treated by that clarifier) / SOR. If you have two parallel trains, each clarifier handles 500 m³/day influent + 500 m³/day RAS split — practical approach: two trains each sized for 500 influent + RAS 500 → clarifier area per train = 1,000 / 30 = 33.33 m². Add 20% safety → ~40 m². Choose circular clarifier diameter: D = sqrt(4×40/π) ≈ 7.14 m → pick 7.5 m dia, depth 3.5 m.
Notes: provide scum removal, RAS pumps sized to maintain RAS rate (100% of influent) and sludge wasting pumps sized for 106.7 kg VSS/day to dewatering.
8) Disinfection tank
Purpose: final pathogen control before filtration and RO feed.
Design:
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If using chlorination (or sodium hypochlorite) for non-potable reuse: typical contact time 15–30 min at designed residual; but since water later goes to RO (which will remove pathogens) you can use modest disinfection. For a conservative design use 30 min contact time.
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Volume = Q_hr × contact time = 20.833 m³/hr × 0.5 hr = 10.42 m³ (for 30 min). Depth 2–3 m → area ≈ 10.42 / 2.5 = 4.17 m² (say 2 × 2.5 m tanks in series).
Notes: If using UV, design as per UV vendor (based on UVT and flow), and you can omit long contact time.
9) Pressure sand filter (PSF) → Activated carbon filter (GAC) → Coagulation + UF/MF
Purpose: final particulate removal and taste/odor/organics polishing to protect RO.
Pressure Sand Filter (rapid sand / multimedia)
Design:
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Filtration rate: 5–10 m³/m²·hr for pressure sand (pick 6 m³/m²·hr conservative).
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Flow for reuse water (after disinfection it's still 500 m³/day incoming, but we intend to send RO feed after coag+UF; we should size filters for RO feed flow which is the permeate target + recycle; practically RO feed = UF permeate ≈ ~400 m³/day permeate but feed to RO is 500? To be safe, filter the full plant flow or the UF feed. I’ll size filters for 500 m³/day = 20.833 m³/hr).
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Area = Qh / rate = 20.833 / 6 = 3.47 m². Use two units in parallel for service/cleaning; each ~2 m² (e.g., 1.6 m dia pressure vessels).
Activated Carbon Filter (GAC)
Design:
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Empty bed contact time (EBCT) typical 10–20 minutes for organics removal. Choose 15 min.
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Volume = Q_hr × EBCT = 20.833 m³/hr × 0.25 hr = 5.21 m³ EBCT. Two parallel vessels, each 2.6 m³. Bed depth 0.8–1.0 m → footprint modest. Service flow velocity and contact time controlled.
Coagulation + UF/MF (pre-RO polishing)
Coagulation: Jar-tests required; typical dosing alum/Fe + polymer before UF.
UF sizing (pre-RO):
Practical UF layout: choose skid with multiple modules (e.g., 20–40 modules) and CIP system. UF retentate returned to sludge handling or to anaerobic digest as co-substrate (if acceptable).
10) Reverse Osmosis (RO) — staged for high recovery
Purpose: produce high-quality permeate for reuse and generate concentrate for ZLD.
Design assumptions & basis:
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Choose conservative RO flux 15 L/m²·hr for challenging feed (low flux prevents rapid fouling).
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Permeate hourly flow = 400 m³/day ÷ 24 = 16.667 m³/hr = 16,667 L/hr.
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Required membrane area = 16,667 L/hr ÷ 15 L/m²·hr = 1,111 m². Add 30% safety / spare → ~1,444 m² total membrane area. (Vendors will quote element counts; e.g., 8-inch elements ~37 m² each — you'd need ~39 elements; multiply by staging and arrays.)
RO high-pressure pump energy: typical range 3–7 kWh/m³ depending on salinity and configurations. Use 4 kWh/m³ baseline → RO electrical = 400 × 4 = 1,600 kWh/day (indicative).
Notes: include antiscalant, acid dosing, high-pressure pump with VFD, permeate polishing line, concentrate recirculation piping.
11) RO concentrate → Evaporator (or Membrane Distillation) → Crystallizer (ZLD)
Purpose: concentrate brine to solids; produce dry salts for disposal — achieve Zero Liquid Discharge.
Design approach (as before):
Evaporator energy estimate:
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Multi-effect evaporators with thermal integration typically ~100–200 kWh(th)/m³ evaporated depending on feed and number of effects. Use 150 kWh(th)/m³ for planning → thermal energy = 90 × 150 = 13,500 kWh(th)/day. (If MD chosen, low-grade heat can be used; MD energy may be less electrical but needs heat input and has scale/maintenance issues.)
Crystallizer & solids handling:
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Expected solids mass: depends on salt concentration in RO brine (unknown without analysis). For conservative mass-balance: if initial dissolved solids (TDS) ~10,000–30,000 mg/L machine, solids produced could be hundreds to a few thousand kg/day. Pilot crystallizer tests will fix numbers. Provide conveyors, rotary drier or filter press to produce dry salt cake, storage shelter and transport arrangements.
Recommendation: integrate UASB biogas into a steam boiler or thermal oil loop to supply part of evaporator heat (biogas from UASB ~569 m³ CH₄/day ≈ 5,653 kWh thermal — realistically supply ~40% of evaporator heat). Evaluate fossil fuel backup and heat recovery from engines/CHP.
12) Solids offsite / disposal
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DAF float: concentrated FOG and solids (3–6% solids) — send for rendering, co-digestion, or dewater and landfill as per regulations.
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Primary/secondary sludge: dewater (centrifuge/belt press) to 20–30% DS, then compost, incinerate or co-process depending on local rules. Expected dewatered cake ~200–500 kg DS/day (estimate). Pilot and lab dewatering tests required.
Quick summary table (key numbers)
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Flow: 500 m³/day
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Oil trap volume: ~42 m³ (2 hr detention)
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EQ tank: 750 m³ (1.5 day) with drum screen (rated ~25–30 m³/hr)
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DAF area: ~3.5 m² (plate pack unit, vendor module for 25 m³/hr)
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Primary clarifier: ~22 m² area → choose 6 m dia × 3.5 m depth (vol ~95 m³)
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UASB: 625 m³ (HRT 30 hr), CH₄ ≈ 569 m³/day
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Aeration tanks (total): 266.7 m³ (two tanks of 133.3 m³ each), total HRT 12.8 hr
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Secondary clarifiers: area per clarifier ~40 m² (choose 7.5 m dia) — two trains
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Disinfection tank: ~10.4 m³ (30 min contact)
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PSF area: ~3.5 m² (2 units parallel)
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GAC EBCT volume: ~5.2 m³ (two vessels)
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UF area (pre-RO): ~500 m² (vendor modules)
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RO membrane area: ~1,444 m² (flux 15 L/m²·hr, 80% recovery)
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RO permeate: 400 m³/day; brine: 100 m³/day
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Evaporator duty (to evaporate 90 m³/day): ~13,500 kWh(th)/day (planning basis)
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Aeration O₂: ~568 kg O₂/day → aeration energy ≈ 227 kWh/day (indicative)
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RO electrical: ~1,600 kWh/day (indicative)
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Pilot / testing required for RO fluxes, UF flux, DAF performance, evaporator energy and crystallizer solids composition.
