Sewage treatment

(Waste water treatment plant RHV, Austria)

Flashback: Traditionally, liquid waste was often dumped into nearby creeks, streams, and rivers. Because of the booming world population and peaking growth rates in GDP, as well as the sheer countless number of toxic chemicals generated by industrial processes, the disposal of such liquid waste has become a pressing problem in the 20th/21st century.
Roman sewer systems, which date back at least to the 6th century B.C., were widely introduced in European cities in the mid-19th century. In the absence of sewerage, waste was often stored in underground cesspools, which were made to leach liquids into the soil but to retain the solid fraction. This approach was followed by the implementation of septic or sewage tanks, in which organic matter ferments and disintegrates. Nowadays, across central Europe, raw sewage is treated before being discharged as effluent. Although the watery fraction after treatment is almost unpolluted, the semil-iquid sludge residuals require extra treatment. Although sludge can be further processed and utilized; e.g. as fertilizer, it has usually buried or dumped at sea (a practice outlawed in the many countries - but still a common practice). Due to the high content of heavy metals and other toxic substances, sewage sludge is unsuitable for agricultural purposes and thus discharged onto landfills.
The importance of saving and maintaining good water quality is clearly recognized by most communities. However the general social perception of water, its use and the ecological problems associated with it, dramatically must be improved. If current rates of consumption by agriculture, industry and domestic users continue, the available water supply will not meet the growing demand. Additional factors such as global warming with the ensuing possibility of unpredictable meso-climatical changes makes this situation even more unsustainable.
This requires that we all, producers and consumers alike, have to make adequate changes to improve current conceptions, usage, and conservation of water related issues. Any industrial application has to be modified to reduce the volumetric and toxicity level of waste-water. Simultaneously a better water management must be enforced: e.g. favouring the use of on-site storm water drainage systems, (currently these freshwater sources are discharged into the gutter, but are sufficient for most industrial applications). On a houshold level, the increasing use of collected rainwater/storm water for toilet and garden application further reduces the overall amount of freshwater consumption.
The availability of dual flush toilet cisterns (5L/10L) is one simple but very efficient example. The amount saved per flush is significant - especially when compared with the most common 10-12L single flush cisterns. Other simple devices regard water saving shower head as well as aerated tab caps. Not only do these devices save water, by controlling flow through a process of aeration, but it also saves energy by reducing the volume of water to be heated. Additionally, flow-control valves can be fitted to showers and taps to restrict flow. Already there are taps are available that mechanically or electronically reduce the water flow while still meeting user requirements. Since washing machines use between 60-200L and a dishwasher between 25-90L per cycle, it follows that prior to purchase, choosing a suitable machine should focus primarily on two factors: the volume of water it uses and the amount of electricity it consumes.

Waste waters from industrial and household processes require appropriate treatment to avoid contamination and eutrophication of water bodies that typically result in algal blooms, sophocating aqueous flora and fauna of limnic waterways and coastal areas.
The most common way to treat waste water is achieved by the mechano-biological treatment. This purification process is applied at the waste water treatment plant in Salzburg (AUT). It is split into four major steps that are described in this page.
In order to maintain a constant gravimetrical gradient the raw sewage must be lifted several meters above ground level. In a preliminary first step, a battery of simple spiral pumps lift the sewage and thus generate a constant and gradual flow through the entire processing plant.

Screw pump

Mechanical purification: Prior to mechanical purification, a coarse collar filter removes large floating objects. Only then can the black waters enter the sedimentation basin. The almost stagnant water body (slow-flowing) allows heavier suspended particles (densities >1g/cm3) to settle down at the bottom of the basin, whereas lighter suspended particles (densities <1g/cm3) accumulate at the surface. Skimmers and bottom cleaners gather these separated fractions, and where possible, treat them separately at the solid waste treatment plant. Excess input, due to storm water rain is collected separately in rainwater basins in order to avoid fluctuations in the flow-rate of sewage dripping through the system.
The bottom bound primary sludge is forwarded to a dehydration chamber.
Þ For further processing steps of the sludge, see below (sewage sludge-section).

Coarse collar

2-Stage Biological purificator: In a 2nd step, the mechanically treated sewage must be stripped of the complex organic and chemical constituents that might interfere with tight environmental regulations of local authorities. This chemo-organic fraction is typically made of in/organic materials, including proteins, cellulose, fats, carbohydrates, phosphates, nitrates, tensides, etc. and are easily "digested" by naturally occurring micro-organisms (mostly bacteria and ciliates).
Practically, purification takes place in two steps:
i) In a series of aeration and settlement basins, the oxygenated aqueous solution boosts the decomposition efficiency of the micro-organisms. These organisms break down, digest the organic matrix, and convert it into inorganic components. The proliferation and increase in microbial biomass does not interfere with purification efficiency as these short living organisms are continuously regenerated, while the decaying parent population gradually settles down at the bottom of the basin.
i) In a follow-up procedure the aerated microbial soup then allowed to settle. Turnover is complete when about 95% of the dissolved organic substances have been used up by these micro-organisms; i.e. their primary food source has become so scarce that, biomass along with mineralised constituents (metabolic by-products) settle down at the bottom, leaving behind cleansed and highly purified water which can safely be reintroduced into the environment or used for other purposes.
The accumulated bottom slick (secondary sludge) is gathered and pumped into dehydration silos, to be disposed off properly later on.


Monitoring and lab analysis

Sludge treatment and fermentation: The sedimented mud of both the primary and secondary stages have to be dehydrated within sealed fermentation tanks, where the water content is lowered to below 50%. Only then, can the condensed sludge be pumped into the fermentation tanks. Anaerobic microbial activity causes the internal temperature to soar to levels where pathogenic strains die (>70C). The decomposing activity of bacteria mineralises biomass into CO2 and CH4. Both the thermal energy as well as the methane gas originating from the fermentation processes are used within the plant to feed heat requiring reactions and to generate electricity by gas-powered turbines.
Þ For gas processing, see also the
landfill-section.
The final composition of the fermented sludge is rich in inorganic phosphorus and nitrogen components and thus theoretically suitable for agricultural purposes. Being slightly enriched with heavy metals and other scarcely degradable and toxic ingredients, it is safer to deposit it permanently at landfills or incinerate it at appropriate facilities.
Þ For final deposition, see the landfill-section.

Inside the fermentation tank

If landfill space is scarce, sewage sludge can be incinerated on site if the appropriate facilities (fluid bed furnace) are available. To burn condensed slick, the solid fraction should at least amount to 37%. This is the minimal level required to maintein a self-combusting reaction; i.e. does not require extra fuel injections. Electro-, sprinklers and charcoal-filters make sure that the generated amount of toxic gases (especially dioxin compounds) is kept below legal limits. The sketch pictured below represents the slick incinerator built in Simmering (Vienna) which is capable of handling up to 60E3 t of enriched sludge per hour. The combustion products, mainly ashes and filter cake are disposed of in landfills or used as additives in the construction industry. This particular sludge incinerator generates so much heat that anything beyond the plants own requirements is then fed into the district heating network of the city of Vienna. By doing so, the all over efficiency of the entire plant increases substantially.
Þ For similar processing steps, see the solid waste (incinerator)-section.
Þ For final deposition, see the landfill-section.

References: Zimmermann E. (1996); Abwasser, Woher-Wohin; Vortrag in Salzburg - AT
Reinhalteverband Grossraum Salzburg; Leitbild (1997); A-5101 Bergheim - AT
Hauptkläranlage Wien, Entsorgungsbetriebe Simmering (1999); A-1110 Wien - AT
For additional information visit the RHV web-site or any other listed below: http://www.rhv-sab.at/rhv/index.html
http://www.ebs.co.at
http://www.abwassertechnik.at/
http://www.brochier.de
www.tve.org/handson/ Hell for Leather - Pakistan
Sewage and Sunshine - India
Green Streets - Ecuador

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