CHARACTERIZATION AND SOURCES OF SLUDGES FROM WASTEWATER TREATMENT

Humans deposit about 70 g per capita per day of solids into wastewater.  With 'garbage grinders', this can reach 100g per day.  The impurities present in the wastewater must either be transformed into innocuous end-products or be effectively separated from the effluent stream.  Impurities which are removed are drawn off as side-streams to the main flow and partially converted into gaseous products.  Treatment and disposal of side-streams is an essential part of the overall treatment process, and frequently they contribute significantly to the total cost of treatment.

In conventional wastewater treatment  works, the main sidestream products, apart from screenings and grit, are the various forms of sludge, comprising  the underflow from sedimentation tanks which effect separation of the greater proportion of the removed impurities.  Treatment and disposal of these sludges is dependent on the volume and characteristics of the sludges produced, which in turn are related to the type of treatment giving rise to the sludge.

The simplest classification of wastewater sludges is based on the process from which they are produced.

Raw or primary sludge  This is drawn from the primary sedimentation tanks.  It contains all the readily settleable matter from the wastewater; plus another 1% collected as scum; it has a high organic content - mainly faecal matter and food scraps - and is thus highly putrescible.  In its fresh state, raw sludge is grey in colour with a heavy faecal odour.  Both colour and odour intensify on prolonged storage under anoxic conditions, leading rapidly to onset of putrefaction and extremely unpleasant odours.  This is often evident in small works when sludge is drawn from the sedimentation tanks into open pits for transfer to the digestion tanks.

Primary sludge accounts for 50-60% of the suspended solids applied.  Primary precipitates can be dewatered readily after chemical conditioning because of their fibrous and coarse nature.  Typical solids concentrations in raw primary sludge from settling municipal wastewater are 6%-8%.  The portion of volatile solids varies from 60% to 80%. 

Trickling-filter humus from secondary clarification is dark brown in colour, flocculent, and relatively inoffensive when fresh.  The suspended particles are of biological growth sloughed from the filter media.  Although they exhibit good settleability, the precipitate does not compact to a high density.  For this reason and because sloughing is irregular, underflow from the final clarifier containing filter humus is returned to the wet well for mixing with the inflowing raw wastewater.  Thus humus is settled with raw organics in the primary clarifier.  The combined sludge had a solids content of 4% to 6%, which is slightly thinner than primary residue with raw organics only.

Waste-activated sludge is a dark-brown, flocculent suspension of active microbial masses inoffensive when fresh, but it turns septic rapidly because of biological activity.  Mixed - liquor solids settle slowly, forming a rather bulky sludge of high water content.  The thickness of return activated sludge is 0.4% - 1.5% suspended solids with a volatile fraction of 0.7 - 0.8.  Excess activated sludge in most processes is wasted from the return sludge line.  A high water content, resistance to gravity thickening, and the presence of active microbial floc make this residue difficult to handle.  Routing of waste activated to the wet well for settling with raw wastewater is not recommended.  Carbon dioxide, hydrogen sulphide, and odourous organic compounds are liberated from the settlings in the primary basin as a result of anaerobic decompostion, and the solids concentration is rarely greater than 4%.  Waste-activated sludge can be thickened effectively by flotation or centrifugation; however, chemical additions may be needed to ensure high solids capture in the concentrating process.

Anaerobically  digested sludge  is a thick slurry of dark - coloured particles and entrained gases, principally carbon dioxide and methane.  When well digested, it dewaters rapidly on sand - drying beds, releasing an offensive odour resembling that of garden loam.  Substantial additions of chemicals are needed to coagulate a digested sludge to mechanical dewatering, owing to the finely divided nature of the solids.  They dry residue is 30% - 60% volatile, and the solids content of digested liquid sludge ranges from 6% to 12%, depending on the mode of digester operation.

Aerobically digested sludge  is a dark - brown, flocculent, relatively inert waster produced by long - term aeration of sludge.  The suspension is bulky and difficult to thicken, thus creating problems of ultimate disposal.  Since decanting clear supernatant can be difficult, the primary functions of an aerobic digester are stabilisation of organics and temporary storage of waste sludge.  The solids concentration in thickened, aerobically digested sludge is generally in the range 1.0% - 2.0% as determined by digester design and operation.  The thickness of aerobically digested sludge can be less than that of the influent, since approximately 50% of the volatile solids are converted to gaseous end products.  Stabilised sludge, expensive to dewater, is often disposed of by spreading on land for its fertiliser value.  For these reasons, aerobic digestion is generally limited to treatment of waste activated from aeration plants without primary clarifiers.