Reading a Wastewater Characterisation Report: The Solids Fractions That Actually Decide Your Process

Why characterisation comes first

The nature of wastewater is the combined characteristic of a complex system made up of water and various impurities, and it is expressed through water quality indicators. Those indicators divide into physical, chemical and biological groups, and together they state which impurities are present and in what quantity. In wastewater treatment, deciding the degree of treatment, selecting the process flow, designing the plant and managing its operation all require a full understanding of that profile; without it, good results are unobtainable.

Physical indicators: colour and solids

Temperature, colour, odour and solids content make up the physical set, and chromaticity and solids content are the two measured most often. Food industry wastewater often contains organic matter, inorganic dyes, biological pigments, inorganic salts and organic additives that colour it, sometimes very deeply. Chromaticity quantifies the degree of colour, generally taking the true colour after removal of suspended solids as the standard, and comparing the sample against a standard coloured solution of known concentration.

Solids matter more. Most impurities in wastewater are solid matter, present in dissolved or suspended form; together they form total solids, including organic compounds, inorganic compounds and various organisms.

The five solids fractions

• Total solids (TS): the residue remaining after a given quantity of water sample is dried at 105-110 degrees Celsius, expressed by weight.
• Suspended solids (SS), or suspended matter: the portion of total solids in a suspended state, containing both organic and inorganic components.
• Volatile suspended solids (VSS): the weight loss of suspended solids after ignition at 600 degrees Celsius, representing the organic fraction. Part of it is biodegradable suspended solids (BVSS) and part is non-biodegradable (NBVSS).
• Non-volatile suspended solids (NVSS): the residual portion of suspended solids after ignition, also known as ash, representing the inorganic fraction.
• Dissolved solids (DS), or dissolved matter: the portion of total solids present in a dissolved state, obtained by drying and weighing the filtrate of a given quantity of water sample.

Using the fractions as a decision tool

TS tells you the total load your solids handling has to carry. SS drives the sizing of screens, sedimentation and flotation. VSS is the number that decides whether biology has anything to eat: a low VSS to SS ratio means most of what you are removing is grit and ash rather than food for biomass, which is exactly the condition that starves an activated sludge plant. Within the volatile fraction, BVSS is the part microbes can mineralise and NBVSS is the part that passes through to sludge. NVSS is what erodes pumps and fills digesters with inert material. DS is what no amount of settling or flotation will touch, so if the permit limits dissolved constituents the flowsheet needs a chemical or membrane step rather than more tanks.

Two plants can receive the same TS and need completely different treatment. Reporting the fractions separately is what makes the difference visible.

Food industry wastewater as a worked example

The food industry uses a wide range of raw materials and produces many kinds of products, so the volume and quality of discharged wastewater vary greatly. What reaches the sewer falls into four groups: solid matter floating in the wastewater, such as vegetable leaves, fruit peel, meat scraps and poultry feathers; substances suspended in it, such as fats and oils, proteins, starch and colloidal matter; acids, alkalis, salts and sugars dissolved in it; and silt and sand carried in with the raw materials along with other organic matter.

The general characteristics are a relatively high content of organic matter and suspended solids, a tendency to putrefy, and generally low toxicity. The main harm is eutrophication of water bodies, which can lead to the death of aquatic animals and fish, promote odour generation from organic matter deposited on the bottom, degrade water quality and pollute the environment.

Turning a waste stream into a carbon source

Food processing and manufacturing wastewater is highly biodegradable, rich in organic matter as a carbon source, and non-toxic, which makes it suitable as a supplementary carbon source for municipal wastewater treatment plants and enables resource utilisation. Mengniu Dairy (Wuhan) Co., Ltd. collects 455,700 tonnes of wastewater each year and, after pretreatment, sends it to the nearby Jinyinhu wastewater treatment plant as a carbon source supplement. Similar applications exist in the beer, baijiu and sugar industries.

The Discharge Standard of Water Pollutants for the Food Processing and Manufacturing Industry (GB 46817-2025), implemented on 1 January 2026, provides policy support for the practice, allowing processing and manufacturing enterprises to negotiate indirect discharge with wastewater treatment plants. This collaborative treatment model reduces treatment costs for enterprises and also cuts carbon emissions, by nearly 400 tonnes per year in the Mengniu case.

Matching treatment to the profile

Biological treatment is generally suitable for this stream, given its high biodegradability, carbon richness and low toxicity. For wastewater with large concentration fluctuations, pretreatment measures such as hydraulic screens and equalisation tanks can be installed for regulation. If effluent quality requirements are very high, or the organic content is very high, two-stage aeration tanks, two-stage biological filters or multi-stage rotating biological contactors can be used, or two biological treatment units combined; an anaerobic-aerobic series biological treatment system is another option.

One practical detail on sludge handling: when selecting polyacrylamide as a sludge dewatering agent for food wastewater, a medium-cationic polyacrylamide is generally chosen. The reasoning mirrors what is done with biochemical sludge at domestic sewage treatment plants, where the sewage also carries a relatively high organic content drawn mostly from catering wastewater, faecal wastewater and bathing wastewater. In terms of the complexity of the wastewater source structure, the two are very similar.