The most widely used method for estimating biomass of forest is through allometric equations. The allometric equations are developed and applied to forest inventory data to assess the biomass and carbon stocks of forests. Many researchers have developed generalised biomass prediction equations for different types of forest and tree species. The allometric equations for biomass estimation are developed by establishing a relationship between the various physical parameters of the trees such as the diameter at breast height, height of the tree trunk, total height of the tree, crown diameter, tree species, etc. Equations developed for single species and for mixture of species give the estimate of biomass for specific sites and for large-scale global and regional comparisons.

@Md. Rafsan Nahian: Thank you but I am looking for biomass estimation methods in wastewater treatment scenario. 

Molecular based techniques such as Real-time PCR can allow you to get a proxy of nitrfication/denifrication activities in activited sludges. For example see : https://link.springer.com/article/10.1007/s00253-006-0805-8

You will easily find other references in literature. Hope it helps, good search !

Dear Nitish,

(ML)VSS is a lump sum measure of organic solids. Typically only a very small fraction of this VSS will be active nitrifying and denitrifying micro-organisms.

That's why we perform respiration analysis on this (ML)VSS in our model based design and operation. For soluble organic and nitrogen compounds we use dissolved oxygen (DO), temperature, oxidation-reduction potential (ORP) and pH as continuous on-line measurements in both bench/lab scale batch test and full-scale operating plants (continuous flow and SBR) along with standard lab analysis of COD, TKN, N-NOx;

Dear Nitish,

as already mentioned above only a small fraction of (ML)VSS will be nitrifiers/denitrifiers. Anyway, if you are just searching for an alternative method to estimate your biomass in activated sludge (since MLVSS measurement requires some time and sample volume), the following rough rules of thumb (!) so far seemed pretty reliable to me:

1) Carbon content in bacterial cells: 50% (dry weight)

2) MLVSS = 0.8 x MLSS

Therefore, if you measure the TOC (for example Hach Cuvette Test):

3) TOC x 2 = MLVSS

4) TOC x 2.5 = MLSS

Only applicable, if your DOC (dissolved organic carbon) is neglectable in comparison to biomass, otherwise you would have to subtract the DOC.

Be careful to shred your sludge flocks, and don't use pipette tips with narrow openings to avoid sludge separation by pipetting.

PS: consistently this study suggests that biomass can also be determined via COD:

https://www.researchgate.net/publication/245023309_A_modified_method_to_determine_biomass_concentration_as_COD_in_pure_cultures_and_in_activated_sludge_systems

Article A modified method to determine biomass concentration as COD ...