TUESDAY, JAN 17, 2023: NOTE TO FILE
Module 2-5
Ecological Engineering for Water
“I intuited that perhaps for the first time in modern history our knowledge of the natural world has reached the stage where we can look into nature and see a coherent body of ecological information there. Life’s inner workings are being decoded and its extraordinary complexity revealed. We are learning that nature has a set of operating instructions of immense significance, which I believe are critical to humanity’s future.”
Dr. John Todd, Healing Earth
BIOLOGICAL WASTEWATER TREATMENT creates a natural ecology of plants, microorganisms, small filtering animals and in some cases fungi. Bacteria living in biofilms around the roots of certain plants break down the long-chain organic molecules in the waste matter into simple non-polluting substances. A small amount of nutrients– nitrogen and phosphorus ((typically less than 5% of the treatment) – is taken up by the plants.
The Biological Processes
Nature makes no waste; the waste products from one species are food for another.
The biological processes at work in biological wastewater treatment can be divided into those in which the micro-organisms breaking down biodegradable material require oxygen and those in which the presence of oxygen is toxic to the the micro-organisms: aerobic digestion and anaerobic digestion.
Anaerobic
Anaerobic processes take place in the absence of oxygen. Anaerobic digestion makes use of naturally occurring micro-organisms and it is essentially a process in which biomass is biochemically transformed by anaerobic bacteria and archaea leaving carbon dioxide (CO2) and methane (CH4). There are 4 key stages: hydrolysis, acidogenisis, acetogenisis and methanogenisis.
• bacterial hydrolysis: bacteria break down (digest) insoluble organic polymers, complex carbohydrates (sugars and amino acids), and make them available in soluble form for other bacteria
• the acidogenic and acetogenic bacteria digest them in the soluble form, transforming them in a two-step process into organic acids and alcohols with release of ammonia and some other gases (see below) methanogenic archaea digest the ammonia to produce methane and CO2.
The stages of anaerobic digestion. Source
These are naturally occurring processes using micro-organisms in the same way that fermentation of food products and alcohol occurs. Anaerobic digestion can also be be used on a small scale to create ‘biogas’ (methane) for cooking and energy. The bio-slurry is used as fertilizer. This is useful in poorer parts of developing countries where women and children spend hours collecting wood with the knock-on effect on land degradation, hours that could be spent in education or economic activities and growing food. We can see that in making choices about achieving SDG 6, we make headway in achieving other SDGs.
Aerobic
Aerobic processes take place where adequate oxygen is present. For example, garden composting of food scraps and other plant material is an aerobic process using naturally occurring micro-organisms that require oxygen. The micro-organisms involved in aerobic digestion in wastewater require oxygen to remove organic carbonaceous waste material. The aerobic digestion takes place close to the surface of wetlands, in gravel or where water is aerated by plants or pumps. It is essentially, the process by which carbon is transformed up a microbial food chain through predation, feeding, and metabolism by protozoans, rotifers, and other microorganisms with CO2 as a by-product.
There are 4 stages
▪ The solubilisation of large organic molecules in water;
▪ The uptake of organic matter (small compounds) from the wastewater by the microorganisms, effectively ingestion;
▪ once the organic material is ingested, it is converted into the bacteria’s own cellular matter through its own metabolic processes and CO2 is released as the by-product; micro-organisms proliferate in a food chain as long as there is oxygen and organic material present.
Because the micro-organisms involved require oxygen, the amount of waste in the water is measured by the demand for oxygen, known as BOD (Biochemical Oxygen Demand). The lower the BOD, the lower the levels of aerobic microbial activity, and, therefore, the lower the levels of waste. BOD is used to measure the effectiveness of the treatment and the safety of the output for health.
5.4. Wetland Ecological Treatment (WET)
WETLAND ECOLOGICAL TREATMENT systems function by harnessing the innate ability of natural wetland ecosystems to absorb and transform the organic nutrients found in wastewater, converting these into plant biomass and soil. WET Systems are a regenerative form of wastewater purification, they are low-entropy systems as they use no fossil fuels or electricity to purify the wastewater. They are solar powered by the ability of plants to absorb solar energy during photosynthesis. The nutrients found in wastewater are mineralised by micro-organisms in the root-zone, which enables the plants to absorb and use them as an input for growth creating biomass - a biological resource which can then be harvested if required.
Source: Biologic Design
WET Systems have a low embedded energy since, unlike conventional reedbed treatment systems, no gravel or plastic aeration/distribution pipes are used in their construction; thus gravel does not need to be quarried and transported to site, and because plastics are not routinely used in the designs WET Systems have a very low embedded energy. Soil in the root-zone, not imported gravel, is the filtration medium and this is, in most cases, already on-site.
The basis of the WET System is a horizontal plug-flow, soil mycorrhizal, multi-species constructed wetland purification and production system, made up of a series of specially designed and constructed earth banks and ponds.
As the wastewater flows through these soil banks it is both purified by microbial action and transpired by growing trees and other plants. The WET System is planted with a wide range of aquatic and marginal plants and a variety of willow types and wetland tree. The from a well-managed WET System, can include one-year old coppiced willow wands for basketry, two-year old willow wands for hurdle making or living willow domes and tunnels, as well as binders used for hedge-laying, and three-year old willow wands which are used for the construction of living willow structures and garden furniture.
Rapidly growing, large, biomass willow types can also be planted and when harvested and seasoned these can be used to fuel simple woodstoves and 'rocket-stoves', ceramic stoves, biomass boilers or combined heat and power (CHP) systems - and so contribute to the energy needs of the site.
WET systems can be built specifically for and in proximity to specific industrial processes. In this example, the WET system was designed for a company making cider. The wastewater is acidic and requires a large natural system to de-acidify as well as treat organic waste and it forms an integral part of the orchards and factory.
Source: (BioLogic Design, 2015)
Module 2, lesson 6
