Suspended Solids and Percent Volatile Matter: 18-Env-B9 Environmental Chemistry and Microbiology, May 2013, Q7

Question 7: Suspended Solids and Percent Volatile Matter (4 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.

Given data
Quantity	Value
Sample volume	25 mL
Tare mass, filter (dry)	1.5325 g
Tare mass, filter + dried solids (105°C)	1.5415 g
Tare mass, filter + ash (after ignition, 550°C)	1.5378 g

Given. Standard Methods 2540D/E gravimetric procedure: mass gained on the filter after drying is total suspended solids (TSS); mass lost on subsequent ignition at 550°C is the volatile fraction of that TSS.

Find. (7.1) TSS concentration, mg/L; (7.2) percent of the TSS that is volatile.

Approach. Take the mass difference for drying to get TSS mass, normalize by sample volume for concentration; take the mass difference between dried and ignited filter to get the volatile (ash-loss) mass, and express as a percentage of the TSS mass.

Suspended solids mass and concentration. $$m_{SS} = 1.5415-1.5325 = 0.0090\ \text{g per } 25\ \text{mL}$$ $$\text{TSS} = \frac{0.0090\ \text{g}}{0.025\ \text{L}} = 0.360\ \text{g/L} = \boxed{360\ \text{mg/L}}$$
Volatile (organic) mass. Ash mass retained after ignition $=1.5378-1.5325=0.0053$ g, so the mass burned off (volatile solids) is $$m_{VS} = m_{SS}-m_{ash} = 0.0090-0.0053 = 0.0037\ \text{g}$$
Percent volatile matter. $$\%VS = \frac{m_{VS}}{m_{SS}}\times100 = \frac{0.0037}{0.0090}\times100 = \boxed{41.1\%}$$

Solids analysis — final results
Quantity	Value
Total suspended solids	360 mg/L
Volatile fraction of TSS	41.1%

Concept & Theory

Topic. Gravimetric suspended-solids and volatile-solids analysis (Standard Methods 2540D/E).

Key relations. $\text{TSS}=\dfrac{m_{\text{dried}}-m_{\text{tare}}}{V_{\text{sample}}}$; $\%VS=\dfrac{m_{\text{dried}}-m_{\text{ash}}}{m_{\text{dried}}-m_{\text{tare}}}\times100$.

Why this works. Drying at 105°C removes only water, leaving all solids (mineral + organic); igniting at 550°C additionally combusts the organic (volatile) fraction, leaving only mineral ash — the mass lost on ignition is therefore the organic content of the solids.

Common pitfall. Reporting %VS relative to the sample volume/TS instead of relative to the TSS mass actually retained on the filter, and forgetting to convert the small gram masses to a per-litre concentration using the correct sample volume in litres.

Source. Metcalf & Eddy, Wastewater Engineering, ch. 2 (wastewater characterization, solids testing).

Question 7 of 25: Suspended Solids and Percent Volatile Matter

Section 1: Chemistry (11 questions, 50 marks)

Question 7: Suspended Solids and Percent Volatile Matter (4 marks)