Volatile and Fixed Solids

Volatile and Fixed Solids

Material that can be volatilized and burned off when ignited at 500 ± 50°C is classified as volatile. In general, volatile solids (VS) are presumed to be organic matter, although some organic matter will not burn and some inorganic solids break down at high temperatures. fixed solids (FS) comprise the residue that remains after a sample has been ignited. thus, TS,TSS and TDS are comprised of both fixed solids and volatile solids. the ratio of the VS to FS is often used to characterize the wastewater with respect to amount of organic matter present.

Estimating Volatile Solids Destruction

The degree of stabilization obtained is often measures by the percent reduction in volatile solids. the reduction in volatile solids can be related either to the srt ot to the detention time based on the untreated sludge feed. the amount of volatile solids destroyed in a high-rate complete-mix digester can be estimated by following empirical equation (liptak, 1974)

V_d = 13.7 ln (SRT_des)+18.9

Where Vd = volatile solids destruction, %

SRT = time of digestion, d (range 15 to 20 d)

Volatile solids destruction estimates can also be made using this table

Estimated volatile solids destruction in high-rate complepe-mix mesophilic anaerobic digestion^a

Degestion time, day	Volatile solids destruction, %
30	65.5
20	60.0
15	56.0

^aThis table was adopted from WEF (1998)

Because the untreated sludge feed can be measured easily, this method is also used commonly. In plant operation, calculation of volatile solids reduction should be made routinely as a matter of record whenever sludge is drawn to processing equipment or drying beds. Alkalinity and volitile acid content should alsobe checked daily as a measure of the stability of the digestion process.

In calculating the bolatile solids reduction, the ash content of the sludge is assumed to be conservative; that is, the number of pounds of ash going into the digester is equal to that being remobed.

Effect of sludge concentration and hydraulic detention time on volatile solids loading factors^a

Sludge concentration	Volatile solid loading factor
	Kg/m3.d
	10 days	12 days	15 days	20 days
2	1.4	1.2	0.95	0.70
3	2.1	1.8	1.4	1.1
4	2.9	2.4	1.9	1.4
5	3.6	3.0	2.4	1.8
6	4.3	3.6	2.9	2.1
7	5.0	4.2	3.3	2.5
8	5.7	4.8	3.8	2.9

^aBased on 70 percent volatile content of sludge, and a sludge specific gravity of 1.02 (concentration effects neglected).
^bHydraulic detention time, d