Bioremediation of Wastewater Effluent Trickling Filters

How to treat sewage has always been a contentious issue… "the only topic besides polemics, upon which men's party spirit got the better of their good sense and even of their regard for truth and justice… the treatment of sewage!"

Primary Treatment Initial sewage is ground and macerated to remove grit and settleable solids. The supernatant has a high BOD Secondary Treatment The contaminants are metabolised by a bacterial consortium as they pass through a trickling filter or an activated sludge stage. BOD lowered before discharge to the rivers Tertiary Treatment Stored in separation ponds or filtered through sand to remove further suspended solids (such as humus)

Slides of plant layout

Primary Settlement - Clarifiers

TRICKLING FILTERS

ORIGINS Trickle Filters came about in the late nineteenth century - it was accepted that effluent with a BOD of 250mg/l (after primary settlement) couldn't be released into rivers. Further treatment was required.

Soaking through soil produced clearer effluent. Excessive soaking would wash out the soil and result in failure (microbes washed away with the soil!) Later experiments tried gravel instead of soil to get round the washing-out problem. Aerobic microbes were responsible for the initial breakdown of the carbonaceous matter.

An optimum thickness of microbes was found to be necessary to ensure adequate flow of liquid/oxygen - excessive growth of microbes caused oxygen diffusion problems.

Pumping the supernatant through the biofilter didn't increase the rate of reaction so the liquid product simply had a higher BOD at the end. Whilst recycling the product was seen to help, it was eventually realised that oxygen was the rate limiting step

By the 1920's engineers had taken a firm grasp of sewage treatment. An agreement was reached that the aim in treatment was to produce "sufficiently clean" output rather than an absolutely pure one. A maximum allowable BOD was assigned and this was to be reached by whatever means.

Water Standards A standard called 30:20 from the Royal Commission on Sewage was established. –30mg/l of settleable solids –20mg/l of BOD Once engineers understood that oxygen was the rate determining stage, different systems were design which could introduce high BOD effluent to the biofilm more efficiently.

Trickling Filters ’s The early years… MICRON SCALE – LOW VOIDAGE SANDS AND SOILS

Pre-1930’s – STANDARD RATE TRICKLE FILTERS MILLIMETRE SCALE – HIGHER VOIDAGE COKE BEDS

POST 1930’s – HIGH RATE TRICKLE FILTER CENTIMETRE SCALE POROUS PLASTICS By the 1930’s high rate trickle filters had been developed with highly porous packing (>95% voidage). The packing was coated in biomass could allow higher rates of flow.

“Flocor” by ICI is an example of such light weight packing – these were utilised in “Flocor Towers”. Flocor had a surface area of 85m 2 m -3 with a density of 38kg m -3 whereas the stone or slag used previously was 40m 2 m -3 and a density of 1100kg m -3.

MICROBIAL CONSORTIUM Of the biological microbial growth on the surface of a substrate heterotrophic carbonaceous bacteria and fungi are the most common. Nitrifying bacteria is common at the bottom of the bed where it anoxic conditions tend to prevail. Photosynthetic algae and bacteria are common on the surface layers.

Protozoa, periodic sloughing or creating anoxic conditions can help to clean away excess biomass. Protozoa, nematodes, rotifiers and annelid worms also feed on the microbes. Weather effects efficiency, slower digestion occurs in the winter since the microbes are less ‘frisky’.

Nomenclature Packing can be stones, or plastic particles ( m 2 /m 3 ) are packed into separate 1m 3 crates. Note that the units for specific surface area is different than used in catalyst work (m 2 /g). A 5mm sphere has: a volume of 4/3*  *r 3 = 6.5x10 -8 m 3 a surface area of 4*  *r2 = 7.85x10 -5 m 2 Hence it’s specific surface area is A/V = 1208m 2 /m 3

Heavy loading can lead to problems due to poor oxygen diffusion throughout the bed Nutrients and oxygen have to be in plentiful supply in order to ensure aerobic metabolism of the carbon material However, if anoxic conditions can be controlled at the lower parts of the filters, this will allow nitrification of the ammonia species to nitrates (NH 3  NO 3 - ).

Piccy…

Trickling filters don’t have to be round…

Possible Configurations A table of accepted flow rates and BOD loading rates is available on the web site. This data can be used with the exercise sheets on trickling bed reactors. web site

Rotating Biological Contactors RBC

Rotating Biological Contactors A biofilm process – discs of 1-3 metres spaced 30mm apart are partially submerged in the supernatant liquid from primary separators and rotated slowly (0.3m/s or 2-6 turns per minute). Spinning the discs faster than this will have negative affects on the microbes. These devices can take around 7g of BOD m -2 day -1. Care must be taken to avoid the discs joining together due to excess biomass build up. If the RBC stops for any length of time then uneven growths of biomass with result which could cause mechanical failure.

Try this at home!

Don’t try at home…