Sewage, Fertilizers, and Biomass: Environmental Impact & Treatment

Posted on Jan 28, 2025 in Biology

Consequences of Sewage and Nitrate Release

Raw sewage contains organic matter, and together with nitrate fertilisers, increases nitrate and phosphate levels in rivers.

Eutrophication

Eutrophication is the ecosystem response to the addition of such substances to an aquatic system and includes:

  • Algal blooms (due to increased nutrients) and a spike in bacteria and microbes feeding on dead algae.
  • Increased biochemical oxygen demand (BOD) by saprotrophic bacteria, leading to deoxygenation (reduced dissolved O2).
  • Death or emigration of oxygen-sensitive organisms, proliferation of pollution-tolerant organisms, reducing biodiversity.
  • Increased toxins, heavy metals, and pathogens, contaminating bathing and drinking water.
  • Decreased water transparency (increased turbidity) stressing photosynthetic organisms and affecting food chains.

Saprotrophic Bacteria in Sewage Treatment

Trickling Filter Beds

  • Trickling filter systems use beds of stone 1–2 meters wide, through which sewage is drained.
  • A biofilm of saprotrophic bacteria on the rocks feeds on the organic matter in the sewage.
  • Cleaner water filters out the bottom into another tank, where bacteria can be removed and water further treated (e.g., chlorinated).

Reed Bed Systems

  • Reed bed systems use artificial wetlands to treat wastewater.
  • As sewage passes through, it is broken down by saprotrophic bacteria living on the root system and in the litter.
  • These microbes use the sewage for growth nutrients, resulting in cleaner runoff.

Biomass as a Fuel Source

Biomass (organic matter) can be used as raw material for the production of biofuels such as methane and ethanol.

  • Ethanol can be produced via fermentation of starch or cellulose by bacteria and enzymes.
  • Methane can be produced from manure, which provides an organic source for anaerobic bacteria to convert the matter into methane gas and CO2.

Methane Generation from Biomass

Principles involved in the generation of methane from biomass:

  • Various organic matter sources can be used, including manure, seaweed, agricultural materials, and sewage.
  • Several groups of bacteria are combined in a digester (a vessel where the chemical reactions will occur):
    • Bacteria first convert the organic matter into organic acids and alcohol.
    • Other bacteria convert these organic acids and alcohol into acetate, carbon dioxide, and hydrogen.
    • Finally, methanogenic bacteria create methane, either through a reaction between carbon dioxide and hydrogen or via the breakdown of acetate.
  • The digester needs to be maintained at a neutral pH (~7), a constant temperature (~35ºC), and kept under anaerobic conditions.