Biological Treatment of Wastewater

Transcription

1 Biological Treatment of Wastewater

2 Why do we treat sewage? Because the law requires it To protect public health by avoiding the spread of water borne diseases To protect the environment by avoiding eutrophication in water courses, algae bloom etc To avoid smelly situation in our backyard 2 Biological Treatment of Wastewater

3 Sewage Composition Suspended solids (e.g. woody components of toilet paper) Biodegradable: proteins, carbohydrates, oils and fats, expressed as BOD [mg/l] Dissolved inorganics: Calcium, sodium and sulphates Surfactants (soap) Pathogens 3 Biological Treatment of Wastewater

4 Sewage Treatment How do we commonly treat wastewater? With bacteria, it is important to know that bacteria can only digest dissolved substances; Physical, fat oil and grease float to the top, solids heavier than water sediment at the bottom; Anaerobic bacteria, classic septic tank; Aerobic bacteria, AWTS; Soil, soil bacteria attack and destroy any remaining pathogens. 4 Biological Treatment of Wastewater

5 Typical Sewage Composition Contaminant Weak [mg/l] Medium [mg/l] Strong [mg/l] BOD COD TOC Total N Total P Fat, oil & grease E. Coli [cfu/100 ml] Desired effluent quality: BOD [mg/l] TSS [mg/l] E. Coli [cfu/100 ml] Secondary Advanced secondary Biological Treatment of Wastewater

6 Thank you bacteria! Bacteria, by the trillions, are the unpaid workers in a wastewater treatment system. However, bacteria don t like change, change in wastewater volumetric flow, temperature, ph, BOD concentration, etc. In large wastewater treatment facilities, industrial or municipal, there is room for frequent human oversight, sophisticated control systems and most importantly a large equalization basin (buffer volume). In the equalization basin the wastewater is well mixed and buffered for volumetric flow and composition, before being fed to the bacteria. With on-site wastewater treatment systems we see minimal controls and no pre-treatment buffer volume which means that there is the potential for shock loading of the bacteria. 6 Biological Treatment of Wastewater

7 Principal Elements of an AWTS Aeration and agitation Sludge separation in clarifier Sludge return to primary treatment (not provided in all systems) Chlorination, when appropriate, when not Pump out to LAA. 7 Biological Treatment of Wastewater

8 Methods of Oxygenation Air has to travel from the atmosphere through the bubble surface into the wastewater. One litre of air in bubbles with a diameter of 1 cm equals 1,910 bubbles with a combined surface area of 0.6 square metres, the same litre of air in bubbles with a diameter of 1 mm equals 1,9 million bubbles with a combined surface area of 6 square metres. Coarse bubble aeration with aeration blower Venturi with aeration pump Fine bubble aeration No active mechanical aeration (e.g. worm farm) 8 Biological Treatment of Wastewater

9 Nitrogen Removal Nitrogen can be removed by alternating anoxic and oxidised conditions. Oxidised: 2NO ! NO 3 - Anoxic: 2 NO e + 12 H + N H 2 O 9 Biological Treatment of Wastewater

10 Common failure of AWTSs Moving parts, pump, aerator, aeration blower (pump failure often due to blocked LAA) Control system exposed to the elements Tripped circuit breaker caused by lightning strike, tripped RCD Tank popping out of the ground after sludge pump out due to failure to fill up the tank(s) Failing LAA 10 Biological Treatment of Wastewater

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15 Thought Bubble In high density developments in non-sewered areas, let clusters of say five properties combine their sewage treatment into one system. The combined financial expenditure may allow for the inclusion of an equalization tank and more sophisticated controls and alarm functions such as remote monitoring. Challenges: Shared responsibilities (body corporate?), landscape elevations hindering gravity flow, siting on a common easement? 15 Biological Treatment of Wastewater