---
title: "Why examine wastewater; solids procedure"
subject: "Sanitary Engineering"
chapter: 5
key: "semester_6/sanitary_engineering:examination-necessity-solids-procedure"
source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/examination-necessity-solids-procedure
site: BCT Engineering
---

# Why examine wastewater; solids procedure

Sanitary Engineering · Chapter 5: Characteristics and Examination of Wastewater

- Asked in: 2069 Bhadra, 8 marks
- Video: [Determination of all Solids in Wastewater/Water | TDS | TS | TFS | TSS | VSS | FSS | VDS | FDS | TSS](https://www.youtube.com/watch?v=WsRxE2IWVmk) — Vishnoo Spotlights
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/sanitary_engineering/examination-necessity-solids-procedure
- Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-5

## Question

Why is examination of wastewater necessary? Describe in detail the procedure of determining fixed, volatile and total solids in the laboratory.

## Short answer

Examination determines nature, strength, and treatment needs. Total solids: evaporate sample at 103-105°C, weigh residue; TS (mg/L) = (W₂–W₁)×10⁶/volume. Then ignite at 550°C: volatile solids = weight loss, fixed solids = remaining residue.

## Answer

#### Why examination of wastewater is necessary
1. To know the **nature and strength** (organic load, solids, nutrients, bacteria) of the wastewater.
2. To **design** the treatment plant and select the treatment process.
3. To **control the operation** and check the efficiency of each unit.
4. To decide the **degree of treatment** needed before disposal to a river or the land, or for reuse.
5. To check the **standards** (effluent limits) and to prevent water pollution and diseases.
6. To study **industrial wastes**, infiltration and illegal connections.

#### Total solids (TS)
A well-mixed sample of 100 mL is poured into a clean, dried, pre-weighed evaporating dish (weight $W_1$). It is evaporated on a steam bath and dried in an oven at 103-105°C for 1 hour, cooled in a desiccator and weighed ($W_2$).

$$
TS\ (\text{mg/L})=\frac{(W_2-W_1)\times10^6}{\text{volume of sample (mL)}}
$$

#### Total fixed and volatile solids
The same dish with the residue is ignited in a muffle furnace at 550 ± 50°C for 15-20 min, cooled and weighed ($W_3$). Organic matter burns away.

$$
\text{Volatile solids}=\frac{(W_2-W_3)\times10^6}{V},\qquad \text{Fixed solids}=\frac{(W_3-W_1)\times10^6}{V}
$$

so $TS = \text{fixed} + \text{volatile}$.

#### Dissolved and suspended solids (for completeness)
A filtered sample (glass fibre filter) is dried at 103°C to give suspended solids, and total dissolved solids = TS − suspended solids. Fixed and volatile solids are found in the same way for each fraction.

Typical values for raw domestic sewage: total solids about 700-1000 mg/L, of which about 50% are volatile.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Which is a reason for examining wastewater?
   A. To decide the degree of treatment and design the treatment plant
   B. To increase BOD
   C. To avoid any need for treatment
   D. To find the water price
   Answer: A. Characteristics govern process choice and effluent compliance. (Key point: Why examine)

2. How is total solids determined?
   A. Ignite at 550°C
   B. Evaporate a sample in a pre-weighed dish, dry at 103-105°C, and weigh
   C. Filter through glass fibre
   D. Settle in an Imhoff cone
   Answer: B. TS = (W2 - W1) x 10^6 / V. (Key point: Total solids)

3. How are volatile solids obtained?
   A. Dry again at 103°C
   B. Weight gained after ignition
   C. Ignite the residue at 550 ± 50°C; the weight lost, W2 - W3, is volatile
   D. Filter the residue
   Answer: C. Organic matter burns away on ignition. (Key point: Volatile solids)

4. What is fixed solids?
   A. (W2 - W3) x 10^6 / V
   B. (W2 - W1) x 10^6 / V
   C. Total solids plus volatile
   D. (W3 - W1) x 10^6 / V, the ash left after ignition
   Answer: D. TS = fixed + volatile. (Key point: Fixed solids)

## Related in Sanitary Engineering

- [Determining total, volatile, settleable solids](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Asolids-determination-total-settleable) — Sanitary Engineering, chapter 5
- [Wastewater examination and sampling](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Awastewater-sampling) — Sanitary Engineering, chapter 5, asked 2×
- [Is treatment necessary? Town wastewater 300 l/s](https://www.bctengineer.com/question.md?key=semester_6%2Fsanitary_engineering%3Atreatment-needed-town-300-ls) — Sanitary Engineering, chapter 6