Modular stack unit — build guide — Designs

Drawings

Build from these drawings—not from marketing photos. Scroll the set; each figure is the measuring source.

Stack section — tiers, cradles, PVC to sump

Double-row bank plan — floor footprint reference

55-gal bay section (optional high-dump variant)

Materials

Gather these before you cut. Optional items use unit optional or qty 0 for notes.

Qty	Unit	Item	Spec	Notes
1	each	Stack frame column 3-tier	Welded 50x50 mm posts + tier shelves + 36 deg open cradles	BSF-MOD-STACK-5G-3
6	each	Cradle flat-bar rings per column	Upper+lower formed flat bar per tier (3 tiers x 2); bend on bucket form	Aluminum or mild steel stock bar; cheapest durable open cradle
1	lot	EPDM edge trim per column	Push-on EPDM (U-channel) on cradle contact edges; no glue	~0.75–1.20 USD/ft sketch; UV/damp resistant
3	optional	Downhill rubber bumper per tier	Bolt-on stop if modules creep under vibration	Drill frame; stronger than adhesive neoprene
3	each	Rack module per column	Adapted bucket: rack bucket kit + lid kit	See /designs/5-gallon-rack/ and /designs/5-gallon-lid/ — not collection pails
0	each	Collection bucket per route	Standard food-grade 5-gal pail no holes	Separate inventory from rack modules
1	each	Rack module spare pool	N+1 adapted buckets per active column	Cheap to replace and re-adapt
3	each	Drain jumper per tier	Food-grade tubing to PVC tee	Quick-disconnect optional at tier swap
1	each	PVC downcomer per column	32-40 mm PVC vertical with tees	Solvent weld; strap to frame post
1	each	Bank leachate header per row	40 mm PVC sloped to sump	Min 1% grade
1	each	Sump	20-60 L sealed or open with vent	Route to treatment - not crop tea without testing
4	each	Foot anchors per column	M10 expansion bolts	Bolt posts to slab
1	each	Roof post tab set	Bolt-on tabs for open-sided roof	Not a sealed building
1	each	Harvest bottle service per tier	1-2 L PET on 28-400 adapter	Reach test for top tier
0	each	55-gal bay row variant	See bay-section.svg + designs/55-gallon/	Optional high-throughput alternate SKU
0	note	Cradle bend form	Fill spare 5-gal with cement; bend bar around outside of filled vessel	Slightly oversized OK; cardboard wrap for clearance; plastic may crack—mass matters
0	note	Avoid expanded-mesh cradle as default	Sags under wet load; traps grime	OK only if framed and lined; rings preferred
0	note	Avoid adhesive neoprene liner as default	Outdoor moisture kills adhesive	Use push-on EPDM instead

Optional spreadsheet download: materials.csv

Design: BSF-MOD-STACK-5G-2 / STACK-5G-3 · BSF-MOD-BANK-5G-4 (assembly)
Alternate: BSF-MOD-BAY-30 / BSF-MOD-ROW-55 (55-gallon horizontal row)
Version: 1.3.0
License: CERN-OHL-P-2.0

Neighborhood- and town-hub scale for operator logistics: modular 5-gallon rack modules in a welded steel frame, stacked two or three tiers high, with PVC leachate routing. Uses the same standard bucket SKU as café pickup routes—but not the same physical bucket (see Collection vs rack).

Cross-cutting rules: /articles/tank-design/. Vessel prep: 5-gallon tilted drum (BSF-5G-TILT-30). Routes and buckets: /articles/waste-stream/.

Why 5-gallon stacks

Concern	55-gal horizontal row	5-gal vertical stack
Full vessel lift	150-200 kg; often two people or hoist	~15-25 kg; one operator
Sourcing	Food-grade drums; harder to swap	Restaurant buckets everywhere
Pickup integration	Must dump bucket into drum	Bucket is the module
Footprint	Wide row	Narrow column, uses height
Drain plumbing	Per-bay hose or gutter	PVC manifold per column

The 55-gallon horizontal bay rack remains a valid high-throughput variant when feed is dumped centrally and drums stay on site—see 55-gallon row variant below.

Concept

  [ roof - open sides ]
        |
   tier 3  (5-gal bucket module, 36 deg)
   tier 2
   tier 1  ----PVC tee----+
        |                  |
   steel frame posts       vertical drop
        |                  |
   operator aisle      header / sump

Part	Role
Steel stack frame	Posts + tier shelves; cradles at 36 deg; bolt to slab
Rack module	Adapted bucket per `5-gallon BUILD`—drilled, access lid, harvest port
Collection bucket	Standard food-grade pail, intact lid, no holes—pickup route only
PVC drain system	Barb/tee from each tier low drain to column downcomer to shared header
Sump	Collection tank; not crop tea without testing

Inventory N+1 rack modules per active column (prepped spares). Collection buckets stay on the route; operator dumps them into rack modules at the hub.

Collection buckets vs rack modules

5-gallon buckets are standard, cheap, widely available, and multi-use. That is the material advantage—not that one pail does every job without modification.

Role	Prep	Stays where
Collection	No drill holes; lidded transport	Restaurant, truck, swap for clean empty
Rack module	Drain weeps, harvest port, access lid, optional drain barb	Cradle on stack frame

Do not mount a restaurant collection bucket on the rack—it lacks drains, harvest geometry, and will leak or fail harvest. Do not return a drilled rack module to a café unless you have a clear re-certification story (default: keep pools separate).

Adaptation workflow (hub workshop or local manufacturer):

Source new food-grade 5-gal pail.
Wash; verify prior contents.
Drill per 5-gallon drawings (drain cluster, underside harvest bulkhead).
Fit bottom-hinged stock lid on head (no latch).
Optional: barb fitting for PVC jumper.
Label as rack module; log in spare inventory.

Damaged units on either side are cheap to replace: new stock bucket + same adaptation steps for rack modules; new collection pail for routes.

Stack geometry (inherits 5-gallon)

Parameter	Value
Vessel	US 5-gal (19 L) bucket or pail; food-grade
Tilt per tier	36 deg +/- 3 deg
Tiers per column	2 or 3 (MOD-STACK-5G-2 / STACK-5G-3)
Tier spacing	Bucket swept diameter + ~80 mm clearance for bottle service
Cradles	Preferred: upper + lower formed flat-bar rings + push-on EPDM edge trim; head stop at upslope (see Open cradle)
Frame load	Rate for 3 x full bucket + substrate per column (~75 kg minimum design load with margin)

Reuse tilt math from 5-gallon (~216 mm leg delta on a single unit at 36°).

Open cradle

Open-top cradles hold the process module at 36° without enclosing the shell. Choose structure and liner for cost, build ease, and damp durability (leachate, humidity, wash-downs).

Structure: flat-bar rings (preferred) vs expanded mesh

Feature	Formed flat-bar rings (upper + lower)	Expanded-metal mesh basket
Cost	Lowest (stock aluminum/steel flat bar; sketch ~$5–12 / cradle)	Higher (sheet cutoffs often ~$15–30+)
Build	Easy — bend around a pipe or bucket form; drill bolt holes	Harder — sharp edges need grinding; sheet bends carefully
Stability	High — two rigid half-rings at rim and lower body	Moderate — thin mesh can sag under ~15–20 kg wet module unless framed
Durability / clean	Excellent — solid bar washes off	Mesh traps grime and wet compost

Default: upper and lower formed flat-bar rings bolted or welded to the tier shelf at 36°. Add a head stop so the bucket cannot slide downhill past the tilt. Do not rely on a floppy mesh basket alone for bank racks.

Anti-slip liner

Option	Verdict
Push-on EPDM edge trim	Preferred — clips onto the bar edge (no glue); UV/weather/damp resistant; sketch ~$0.75–1.20 / ft
Self-adhesive neoprene strips	Cheap short-term; adhesive fails outdoors with moisture, heat, and leachate
Bolt-on rubber bumpers	Strong mechanical lock against downhill slide; needs extra drilled holes

Default liner: push-on EPDM edge trim on the contact edges of both rings. Optional bolt-on bumper at the downhill (low) side if modules still creep under vibration.

Bending form (field mandrel)

You do not need a machined mold. A little oversized radius is better than tight—restaurant pails vary, and a loose cradle plus EPDM still holds at 36°.

Preferred DIY form: take a spare food-grade 5-gal bucket, fill it with cement (or packed wet sand if cement is scarce), and bend the flat bar around the outside of the whole vessel—not a cement plug pulled out of the shell. The plastic may crack or UV-age; the mass inside keeps the cylinder from ovalizing while you lever or hammer the bar. Leave the filled bucket as a permanent shop mandrel.

Tip	Why
Wrap 1–2 layers of cardboard or carpet on the form before bending	Intentional clearance for fat/ribs/non-standard ODs
Mark upper vs lower ring height on the form	Rim ring vs lower-body ring sit at different stations
Deburr bar ends before EPDM	Trim seats cleaner; less cut risk
Optional later: steel pipe ~ bucket OD, or plywood disks	Faster for batch builds—not required for first racks

Empty thin-wall buckets alone crush under bending force—fill them. Expanded-mesh still is not the default cradle even with a good form.

  head (high) →                    ← tail (low)
       [ head stop ]
            U  flat-bar ring (upper, near rim) + EPDM
           / \
          |   |  5-gal process module @ 36°
           \ /
            U  flat-bar ring (lower) + EPDM

Standalone welded stands can use the same ring+EPDM recipe; cinder-block patio hacks still pad with hose scrap / cardboard (see 5-gallon stand options).

PVC drainage

Each tier: low-quadrant drain from 5-gal module with hose barb or bulkhead facing the frame centerline.
Flexible short jumpers (food-grade tubing) tier to PVC tee on vertical downcomer.
Downcomer: 32 mm (1-1/4 in) or 40 mm (1-1/2 in) PVC along frame post to bank header.
Bank header: sloped PVC pipe (min 1% grade) to sump—open top or sealed with vent.
Service: flush header monthly; inspect tees when swapping modules.

Keep leachate separate from harvest bottles and frass—see /articles/leachate/.

Tray / sump SOP

Layout	Capture	Empty / clean
Standalone 5-gal	Shallow pan under low-end drain cluster	Slide out daily–weekly; rinse; never pour into harvest bottles
Modular column	Tier jumpers → PVC drop → shared sump/tray at header end	Dump sump on schedule; flush header monthly; keep aisle dry
Clearance	High end free for hanging bottle + hanging-open lid	Do not raise sump under the head

Municipal pads still use many BANK-5G-8 copies; each bank’s header/sump is part of the facility sketch—not a different larva machine.

Stack SKUs

ID	Layout	Typical use
BSF-MOD-STACK-5G-2	1 column, 2 tiers	2 modules · ~1.6 kg/day mid
BSF-MOD-STACK-5G-3	1 column, 3 tiers	3 modules · ~2.4 kg/day mid
BSF-MOD-BANK-5G-4	4 columns x 3 tiers	12 modules · ~10 kg/day mid · ~2.2 m²
BSF-MOD-BANK-5G-8	8 columns x 3 tiers	24 modules · ~19 kg/day mid · ~4.4 m²

Scale the hub by adding stack columns under one roof—not enlarging a single vessel.

Capacity planning

Scale by multiplying units → columns → racks. Constants below are planning defaults—replace with field measurements from your hub.

Constants (planning defaults)

Symbol	Value	Meaning
`V_nom`	19 L	Nominal 5-gal shell per rack module
`V_act`	8.5 L	Active wet bed per module (~45% of shell at steady state)
`F_lo`	0.5 kg/day	Wet feed per module — conservative (learning / dilute coffee blend)
`F_mid`	0.8 kg/day	Wet feed per module — mid (mixed trim, hub planning default)
`F_hi`	1.2 kg/day	Wet feed per module — aggressive (dense, well-managed)
`T`	3	Tiers per column (MOD-STACK-5G-3)
`P_col`	0.42 m	Column pitch along aisle
`D_row`	0.42 m	Row depth (aisle to back of column)
`W_aisle`	0.90 m	Aisle width

Modular formulas (modules = columns × tiers):

V_nom_total  = modules × 19 L
V_act_total  = modules × 8.5 L
F_lo/mid/hi  = modules × 0.5 / 0.8 / 1.2 kg/day

Substrate volume ≠ daily throughput. Larvae limit kg/day; stored bed volume is biology holding capacity.

Per unit — one rack module

One adapted 5-gal bucket in a cradle (5-gallon prep). Smallest counting unit. Feed profiles (coffee vs animal tailings): 5-gallon capacity.

Metric	Value
Modules	1
Nominal shell (`V_nom`)	19 L
Active wet bed (`V_act`)	~8.5 L
Wet feed/day	0.5 / 0.8 / 1.2 kg (lo / mid / hi)
Plan footprint share	Stacked in column — see column row

Per column — MOD-STACK-5G-3

One vertical stack (3 tiers, single plan footprint). Design ID BSF-MOD-STACK-5G-3. Two-tier variant STACK-5G-2: multiply unit row by 2 instead of 3.

Metric	Formula	Value (3 tiers)
Modules	`T`	3
Nominal volume	`T × 19 L`	57 L
Active wet bed	`T × 8.5 L`	~26 L
Wet feed/day (lo / mid / hi)	`T × F_*`	1.5 / 2.4 / 3.6 kg
Plan footprint	`P_col × D_row`	~0.42 × 0.42 ≈ 0.18 m²

Per rack — assembly of columns

A rack is a welded frame section operators treat as one install unit. Two reference SKUs:

MOD-BANK-5G-4 — single-sided row (4 columns × 3 tiers, one aisle + columns):

Metric	Formula	Value
Columns	4	4
Modules	`4 × T`	12
Nominal volume	`modules × 19 L`	228 L
Active wet bed	`modules × 8.5 L`	~102 L
Wet feed/day (lo / mid / hi)	`modules × F_*`	6 / 10 / 14 kg
Processing footprint	`(4 × P_col) × (W_aisle + D_row)`	~1.68 × 1.32 ≈ 2.2 m²

MOD-BANK-5G-8 — double-row, back-to-back (4 + 4 columns × 3 tiers, aisles both sides). See drawings/stack-bank-double-row.svg.

Metric	Formula	Value
Columns	`4 + 4`	8
Modules	`8 × T`	24
Nominal volume	`modules × 19 L`	456 L
Active wet bed	`modules × 8.5 L`	~205 L
Wet feed/day (lo / mid / hi)	`modules × F_*`	12 / 19 / 29 kg
Processing footprint	`(4 × P_col) × (2×W_aisle + 2×D_row)`	~1.68 × 2.64 ≈ 4.4 m²
With sump / margin (~15%)		~5.1 m²

Scale racks: N double-row blocks → N × 8 columns, N × 24 modules, N × 4.4 m² processing pad (before margin).

Summary ladder (mid feed `F_mid`)

Level	SKU example	Modules	Active bed	Feed/day (mid)	Footprint
Unit	1 bucket	1	~8.5 L	0.8 kg	(in column)
Column	STACK-5G-3	3	~26 L	2.4 kg	~0.18 m²
Rack (single row)	BANK-5G-4	12	~102 L	~10 kg	~2.2 m²
Rack (double row)	BANK-5G-8	24	~205 L	~19 kg	~4.4 m²
2× double-row racks	2 × BANK-5G-8	48	~410 L	~38 kg	~8.8 m²

Per m² floor (double-row rack basis)

Divide MOD-BANK-5G-8 totals by 4.4 m²:

Metric	Per m²
Modules	~5.4 / m²
Nominal volume	~103 L/m²
Active wet bed	~46 L/m²
Wet feed/day (lo / mid / hi)	~2.7 / 4.3 / 6.5 kg/(m²·day)

Reference feed examples (mid case)

Hub layout	Modules	Mid feed/day
1 column (3 tier)	3	~2.4 kg
MOD-BANK-5G-4	12	~10 kg
MOD-BANK-5G-8	24	~19 kg
10 cafés × ~10 kg wet SCG	—	~100 kg → ~5 × BANK-5G-8 (café SCG claim)
~50 m² double-row pads	~290 modules	~230 kg
~100 m² double-row pads	~580 modules	~460 kg

Not pinned research claims—ramp feed to observed consumption. See substrate moisture target.

Sanity checks:

Larval biomass must match feed—overfeeding causes anaerobic beds (aerobic vs house-fly claim).
Leave ~20% module spare offline when sizing racks.
Collection-bucket dump/rinse zone is extra floor beyond processing footprint.

Hub deployment reference

How far one modular install typically goes before you leave this design for a multi-bank facility. Uses F_mid (0.8 kg/module/day) unless noted. Town / dump-adjacent programs: /designs/municipal/.

Homestead — one `BANK-5G-8`

A family homestead that converts farm offal, spoiled fruit, and slaughter trim into larvae for pigs/ducks/chickens typically lands in a 10–20 kg/day planning band (peaks higher on slaughter weeks).

Item	Planning value
Steady wet input	10–20 kg/day (peaks 25–35 kg/day in strong offal/fruit season)
Target hardware	1× `BANK-5G-8` (24 modules)
Mid capacity (`F_mid`)	~19 kg/day
Animal-tailings band (`1.2–1.8` kg/module/day)	~29–43 kg/day — common once beds are mature (5-gallon feed profiles)
Processing pad	~4.4 m² (+ ~15% → ~5 m²)
Under-roof footprint (rack + tote staging + aisle)	~12–16 m²
Offline spare	~20% modules (~5 buckets)

If steady feed stays below ~15 kg/day, a single BANK-5G-4 (12 modules, ~2.2 m²) is enough until volume is proven. Typical phased path: 3–4 standalone tilted drums → migrate into BANK-5G-4 → expand to BANK-5G-8.

Neighborhood café hub

Ten cafés × ~10 kg/day wet SCG ≈ 100 kg/day ≈ ~5× BANK-5G-8 (café SCG claim). Still a small multi-bank roof bay—not a municipal pad.

Beyond roughly a few double-row banks, size and site the facility under municipal (per 1,000 households, town dump concentrator, aisle fans, crew training).

Animal-tailings hubs may size closer to F_hi or field-measured 1.2–1.8 kg/module/day—that reduces rack count for the same kg/day, but only after logs justify it.

Tools

Welder, drill, level, inclinometer, PVC cutter/deburr, solvent weld supplies, torque wrench for cradle hardware.

Build sequence

1. Fabricate stack frame

See drawings/stack-section.svg, drawings/stack-bank-plan.svg, and drawings/stack-bank-double-row.svg.
50x50 mm square tube posts; tier shelves from angle or tube; cross-brace columns.
Weld or bolt open cradles at 36 deg on each shelf: upper + lower flat-bar rings with push-on EPDM edge trim; head stop lip per slot (see Open cradle).
Prefer rings over expanded-mesh baskets for bank racks (mesh sags and traps grime).
Verify tier spacing: bucket clears hanging bottle below with ~50 mm gap minimum; lid can swing open downward.
Bolt frame to slab; plumb posts.

2. Pre-fit 5-gallon modules

Build each bucket per 5-gallon build guide:

Stock lid hinged on bottom edge of the open end (hangs open; no latch)
Harvest bottle hanging under high end (~60 mm below head ring)
Low-point drilled weeps (no mesh); barb fitting on drain zone for PVC jumper

Label buckets (Stack 2 / Tier 3). Keep identical barb positions across spares.

3. Install PVC plumbing

Dry-fit downcomer and tees before mounting buckets.
Solvent-weld PVC; support pipe every ~600 mm.
Slope header to sump; no dead legs.
Water-test with tier buckets filled—no leaks at barbs.

4. Drop-in and commission

Seat bucket in cradle (EPDM-lined rings); chime against head stop.
Confirm module cannot slide downhill; add optional bumper if needed.
Connect drain jumper to tee (quick-disconnect optional).
Attach harvest bottle; verify tilt.
Leak-test; then biological startup tier by tier (bottom up). Seed pack: /guides/setup/#seed-a-new-bucket (food + torn cardboard; larvae optional).
Train operator: dump collection buckets into rack modules; return clean empties to route.

5. Daily feed (operator SOP)

Receive full collection buckets from route.
Dump contents into rack module feed opening (or top up through access lid).
Rinse collection pail if program requires; leave on route inventory.
Return clean empty collection buckets to pickup points.

6. Rack module swap (maintenance SOP)

Stop feeding target rack module; harvest prepupae.
Disconnect drain jumper and bottle.
Lift adapted bucket out (one person).
Seat spare prepped rack module; reconnect.
Wash fouled module offline; re-drill/adapt if shell is still good, or retire.

55-gallon row variant

For central dump sites where operators empty 5-gal pickup buckets into larger vessels, the horizontal 55-gallon bay rack may still fit:

Design IDs: BSF-MOD-BAY-30, BSF-MOD-ROW-55 (3/6/12 bays)
Drawings: drawings/bay-section.svg, drawings/row-plan.svg
Vessel: removable 55-gal HDPE drum per 55-gallon

Use when throughput per footprint matters more than bucket-for-bucket pickup integration.

Maintenance

Weekly: inspect PVC joints, bottle threads, cradle stops, and EPDM trim (replace if torn).
Monthly: flush drain header; check lower-tier load on frame (no rack deflection).
Replace cracked buckets at drill points; keep spare modules dry-stacked nearby.
Shade/roof mandatory—see /articles/tank-materials/.

Field validation (v1.1.0)

Tier spacing with your bottle size and lid swing
One-person lift height for top tier (stool vs lower-only feeding)
PVC clogging rate with your feedstock fines
Frame deflection with three full tiers after 30 days

Report via /contribute/ or a registered community pilot.

Files

File	Purpose
`bom/materials.csv`	Stack frame + PVC BOM
`drawings/stack-section.svg`	Side elevation - 3-tier column + PVC
`drawings/stack-bank-plan.svg`	Plan - multi-column bank
`drawings/bay-section.svg`	Alternate: 55-gal bay section
`drawings/row-plan.svg`	Alternate: 55-gal row plan
`5-gallon/`	Bucket module prep

References

/articles/waste-stream/ · /articles/tank-design/ · 5-gallon