Composting Recipe — Broiler Litter, Sawdust and Water: 18-Env-B6 Agricultural Waste Management, May 2018, Q2

Question 2: Composting Recipe — Broiler Litter, Sawdust and Water (15 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Part (a) — recipe.

Given. Table A.1 "Average" rows, attached appendix:

Table A.1 — feedstock properties (average values)
Material	%N (dry wt.)	C:N ratio	Moisture (% wet wt.)
Broiler litter	2.7	14	37
Sawdust	0.24	442	39

Find. The mass ratio of broiler litter : sawdust : water giving a blended C:N of 30:1 and 60% moisture (wet basis).

Approach. Convert each feedstock's C:N and %N to an equivalent %Carbon (dry basis), blend the two solids on a DRY-MASS basis to hit the target C:N, then add water (no carbon or nitrogen) until the wet-basis moisture reaches 60%.

Step 1 — back out %Carbon (dry basis). Since C:N = %C⁄%N (dry weight to weight), %C = (C:N)×%N. $$\%C_{\text{litter}} = 14 \times 2.7\% = 37.8\%, \qquad \%C_{\text{sawdust}} = 442 \times 0.24\% = 106.1\%$$ The sawdust figure is a data-quality artifact of the table (flagged in part b) — carried forward as printed, per "use attached Table A-1."
Step 2 — blend dry solids for C:N = 30:1. Let r = (dry mass litter)⁄(dry mass sawdust), taking 1 dry-mass unit of sawdust as the basis: $$30 = \dfrac{r(0.378) + 1.0608}{r(0.027) + 0.0024}$$ Solving, $r = 2.289$ — about 2.29 kg dry broiler litter per 1 kg dry sawdust.
Step 3 — convert to wet (as-received) mass. Dividing by each material's own solids fraction (1−moisture): $$W_{\text{litter}} = \dfrac{2.289}{1-0.37} = 3.633\ \text{kg wet}, \qquad W_{\text{sawdust}} = \dfrac{1.000}{1-0.39} = 1.639\ \text{kg wet}$$ Before water, this blend already sits at moisture $= 1-\dfrac{2.289+1.000}{3.633+1.639}=37.6\%$ — well short of the 60% target.
Step 4 — add water to reach 60% moisture. Water adds no dry solids, so the blend's 3.289 kg dry solids must become 40% of the final wet mass: $$W_{\text{total, final}} = \dfrac{3.289}{1-0.60} = 8.222\ \text{kg}, \qquad W_{\text{water}} = 8.222-(3.633+1.639) = 2.950\ \text{kg}$$
Step 5 — state the recipe on a practical basis. Scaling Step 4 so broiler litter = 1000 kg wet, as-received: $$\boxed{1000\ \text{kg broiler litter} \;:\; 451\ \text{kg sawdust} \;:\; 812\ \text{kg water} \quad (\text{total} \approx 2263\ \text{kg wet mix})}$$ a wet-mass ratio of roughly 2.2 : 1 : 1.8 (litter : sawdust : water).

Final Results — Question 2(a)
Quantity	Value
Dry-mass ratio, litter : sawdust	2.29 : 1
Recipe (wet basis, per 1000 kg litter)	1000 kg litter : 451 kg sawdust : 812 kg water
Wet-mass ratio, litter : sawdust : water	2.2 : 1 : 1.8
Resulting C:N ratio (check)	30.0 : 1
Resulting moisture content (check)	60.0%

Part (b) — concerns with this mixture.

Data quality of the sawdust entry. Table A.1's average sawdust row implies %C = 106%, a physical impossibility, because the handbook's own note says these values are pooled from different literature sources and "should not be considered as the true ranges or averages" — %N and C:N were not necessarily measured on the same samples. A prudent designer would seek a paired, internally-consistent data point or a fresh lab test rather than trust the average outright; a more typical wood %C≈50% moves the recipe to roughly 1000 kg litter : 957 kg sawdust : 1078 kg water — different quantities, though the qualitative conclusion (substantial bulking agent, large water addition) is unchanged.

Water addition is a large fraction of the mix (36% by mass). Hauling and uniformly incorporating >800 kg water per tonne of litter is a real handling burden; in practice a composter would substitute some water with a wetter co-feedstock (liquid manure, leachate) where available.

Broiler litter's own C:N is only an ESTIMATE (table footnote a: "estimated from ash or volatile solids data"), so the design C:N of 30:1 carries more uncertainty than the arithmetic alone suggests.

Bulk density / mixing behaviour. Sawdust (410 lb/yd³) is far less dense than broiler litter (864 lb/yd³); by volume the pile will look far more sawdust-dominated than the 2.2:1 wet-mass ratio suggests, which can mislead field crews mixing "by eye."

Pathogen and ammonia-loss risk before water is added. The intermediate 37.6% moisture blend sits below the ideal composting range (50–65%), and fresh, N-rich litter is prone to ammonia volatilization if it sits un-composted at that stage — water should be blended in promptly, not staged separately.

Check: Table A.1's sawdust "average" row (%N=0.24, C:N=442) multiplies to 106% carbon, which is not physically possible. The boxed recipe uses these values literally (as instructed), consistent with solving printed exam data as given and flagging the anomaly rather than silently substituting a different number.

Topic: Two-ingredient composting recipe design (C:N and moisture balance).

Key relations: %C = (C:N)×%N (dry basis); blended C:N = (Σ dry-mass×%C) ⁄ (Σ dry-mass×%N); moisture balance $W_{final}=\dfrac{\text{dry solids}}{1-M_{target}}$.

Why this works: Composting microbes need roughly 25–35 parts carbon per part nitrogen and 50–65% moisture to stay aerobic and active; the two-material blend controls C:N through dry-mass proportioning (moisture-independent), then water is the last, independent adjustment because it carries neither carbon nor nitrogen.

Common pitfall: Blending on a WET-mass basis instead of dry-mass gives the wrong C:N, because %C and %N are defined on dry matter; always convert to dry mass first, then convert the answer back to wet mass for the field recipe.

Source: Rynk et al., On-Farm Composting Handbook (NRAES-54), Table A.1 and recipe-development chapter.

Question 2 of 16: Composting Recipe — Broiler Litter, Sawdust and Water

Question 2: Composting Recipe — Broiler Litter, Sawdust and Water (15 marks)