TUESDAY, JAN 17, 2023: NOTE TO FILE
Module 2-Appendix
Appendix
Extra details for enthusiasts on the technical aspects of sizing and constructing wetlands and on the biochemical processes involved. This material is not obligatory as part of the module and does not feature in the assessments.
Nitrification
Nitrification is the process by which nitrifying bacteria convert ammonia to nitrate. The process happens in two distinct steps. Nitrosomonas sp. oxidizes the ammonia to the intermediate product, nitrite. Nitrobacter sp. converts the nitrite to nitrate. Nitrifying bacteria are considered autotrophic bacteria, or bacteria that utilize CO2 as their carbon source for growth.
The nitrifying bacteria are environmentally sensitive organisms. A variety of environmental factors can inhibit their growth. These inhibiting factors include high ammonia concentrations, low temperatures, pH outside of the 6.5 to 8.6 range and low dissolved oxygen (<1 mg/l). The pH of the system can change dramatically due to the nitrification process. The alkalinity in the wastewater is consumed at a rate of 7.14 mg of alkalinity, as CaCO3, for every milligram of ammonia oxidized. If there is not a sufficient alkalinity concentration in the wastewater, the pH will be depressed as the ammonia is oxidized. Alkalinity addition is not typically necessary for domestic sewage treatment and is not expected to pose a problem for a typical ecovillage treatment system.
Nitrogen cycle in wetlands
The rate of nitrification is also significantly affected by the fraction of nitrifiers present in the wastewater. When the concentration of biodegradable organics, measured as BOD5, is high, the heterotrophic bacteria, or bacteria that use organic carbon as a carbon source, dominate the bacterial population. Typically, when the BOD5 is reduced below 80 mg/l the population of nitrifying bacteria is large enough to begin the nitrification process. For this reason, nitrification happens in the last stages of treatment. For systems requiring enhanced ammonia treatment, consideration of recycling internally to a gravel mound is excellent for nitrification, being primarily a simple aerobic trickling filter reactor towards the end of the treatment train. Some nitrification will also take place in the constructed wetland.
Denitrification
Denitrification is the biological conversion of nitrate to nitrogen gas, nitric oxide or nitrous oxide. These compounds are gaseous compounds and are not readily available for microbial growth; therefore, they are typically released to the atmosphere. Nitrogen gas makes up over 70% of atmospheric gases, thus the release of N2 to the atmosphere is benign.
Biological denitrification is an anaerobic respiration reaction in which nitrate (NO3) is reduced. Denitrifying bacteria are aerobic autotrophs or heterotrophs that can switch to anaerobic growth when nitrate is used as an electron acceptor (Bitton 1994). Denitrification can occur by two pathways. The dissimilative nitrate reduction pathway requires anoxic conditions and results in the liberation of nitrogen gas from the water column (Reed et al. 1988; Madigan et al. 1997). Under aerobic conditions denitrification results in the assimilative pathway or accumulation of nitrogen into biomass (Bitton 1994; Madigan et al. 1997). It is desirable to encourage the dissimilative pathway of denitrification so that nitrogen may be completely removed from the system in gaseous form rather than simply recycled through the system in biomass. In order for this to occur, there must be insufficient molecular or dissolved oxygen present so that the bacteria use the nitrate rather than the oxygen. The rate of the denitrification reaction is relatively fast when there is no free oxygen present (< 0.5 mg/l is ideal). The denitrification rate drops to zero when the dissolved oxygen level reaches 2.0 mg/l.
Source: Bowden, William B., 1987. The Biogeochemistry of Nitrogen in Freshwater Wetlands. Biogeochemistry, Vol. 4, No. 3, pp. 313-338.
The denitrification process partially reverses the effects of the nitrification process in regards to alkalinity concentration. For every milligram of nitrate reduced to nitrogen gas, around 3.57 mg of alkalinity, in the form of CaCO3, are created.
Denitrifiers also require the presence of organic matter (carbon source) to act as an electron donor (see carbon and nitrogen cycle diagrams below). The presence of a carbon source is the primary determinant of denitrification rates in water (Weier et al. 1994). Sources of such electron donors may be raw wastewater, methanol, and decomposing organic matter (Bowmer 1987; Bitton 1994; Weier et al. 1994). Alternatively, organic molecules resulting from solubilisation can be used, but are less efficient. If the main engine of dentrification is a subsurface flow constructed wetland, raw wastewater and decomposing plant matter provide the necessary carbon source for denitrification.
Denitrifiers belong to several genera including Pseudomonas, Bacillus, Spirillum, Hyphomicrobium, Agrobacterium, Acinetobacter, Propionobacterium, Rhizobium, Cornebacterium, Cytophata, Thiobacillus, and Alcaligenes. However, the most wide spread in water and wastewater are Pseudomonas fluorescens, P. Aeruginosa, P. denitrificans and Alcaligenes sp. (Smith et al. 1994; Bitton 1994). These organisms are ubiquitous and commonly found in natural soils and wetland environments.
Sizing of Wetlands for Sewage Treatment Following Pre-Treatment in Septic Tanks
CONSTRUCTED WETLANDS are sized using temperature dependent process equations. For an ecovillage application, the following table will provide a guide. This is only a guide and the process calculations should be checked by a qualified engineer before construction.
A person equivalent (pe) gives the daily wastewater influent to the treatment system. It is 60 grams of BOD per day, which is typically 200 litres with a BOD concentration of 300 mg/l (parts per million). For the same daily flow, typically the ammonia will be ~35 mg/l and the Total Suspended Solids (TSS) 250 mg/l.
As will be seen, the higher the temperature of the wastewater, the greater is the biological activity and so the more the treatment. Thus, a wetland in a tropical country will be smaller for the same level of treatment than one in a cold country.
The chart below gives the size of the working area of the wetland (between the manifolds) per person (pe) for four influent sewage water temperatures. The effluent treatment concentrations are also shown. It is recommended that at least 30% should be added to the size for berming of the edges, access and so on. Typically constructed wetlands are built 50 m to 100 m from the nearest house, although when properly built there is no odour. For very small wetlands (less than 50 people), the actual footprint should be calculated as berming can be a greater percentage of the total area required.
Further (Optional) Vertical Flow Reed Bed to Treat Septage
SLUDGE IS THE HIGH SOLIDS, organic material that is a discharge from aerated tank-based systems. It is mainly composed of dead bacteria and some inorganics, such as sand and grit.
Septage is the sludge that settles at the bottom of septic tanks. Sludge and septage usually has a BOD of greater than 10,000 mg/l and often a strong odour. An economical way of treating sludge and septage is with a vertical flow reed bed. The bed is a simple planted sand filter, installed inside an impermeable liner. Leachate is the high BOD, but low TSS wastewater that collects at the low point in the bed. Typically, it is pumped to a wastewater treatment facility, if nearby, or alternatively to a specially built horizontal flow sub-surface constructed wetland
The pores in the sand of the Vertical Flow Reed Bed are kept open by the plant stems, shoots and roots. The plants improve de-watering by seeking out moisture, and by keeping the septage open to air, as the reeds move with the wind. At the same time, roots treat biological slime in the sand.
Sludge, as organic solids, will accumulate over time on the surface of the bed. A typical loading rate is 50 kg dry weight of sludge per m2 of reed bed per annum. Continuous exposure of the sludge to air promotes the decomposition and oxidation of the sludge. The volumes of sludge will be reduced between 90 to 98%. The principal form of reduction is de-watering. Mineralization is also significant (Kim 1993, Nielsen 1993, Reed et al 1995).
The bed is designed with a freeboard, which allows the sludge to accumulate for many applications. The bed can be designed for clearing every ten years. If and when clearing becomes necessary, the accumulated material is scraped off the surface of the bed. The treated sludge, which is now a stabilized composted material, is suitable for land application, particularly for landscaping and forestry. Sludge and septage treatment beds typically have odours generated during the loading of the bed. However, within 15 minutes to an hour, obnoxious odours are replaced by an earthy smell, similar to that of the soil and leaf litter on a forest floor. During the loading period, most of the odours are trapped by the leaves and plant stems. Most people cannot smell any odour when 50 to 100 metres down wind. The bed can be sited among trees to reduce further the possibility of unpleasant odours. No buildings should be within 100 metres of a septage treatment bed. Aeration of sludge or septage before bed application will stabilise the material and reduce odours significantly.
Food Chain Reactors provide high levels of treatment performance. Here the average annual BOD was 8.2 mg/l with a highly variable influent, demonstrating the robustness of the system.
For locations where land is expensive and there is a significant wastewater loading, a lagoon system or FCR is likely to be most suitable technology. If there is the room to use a lagoon, this will be less expensive than a FCR. In most major cities where the ecovillage is large, the more highly engineered FCR or similar will be the most cost-effective choice. (Note: Foodchain Reactors (FCR) used to be referred to as Feedback Batch Reactors (FBR) in the past.)
The table below gives some approximate numbers and outlines comparisons. Costs will vary with the region. For example, where gravel prices are low, SSFCWs will be less expensive. Low excavation prices will favour lagoons and so on.
Note: this table offers only a guideline for initial design considerations at the community scale. Once the decision what system to build is taken, the guidance of an expert and detailed engineering and costing is necessary for each project.
The Energy and Carbon Footprint of Sewage Systems
THE CHART TO THE SIDE gives an indication of the energy used per 4,000 m3/d of sewage treated (in US terms, this is 1 MGD). Constructed wetlands use the lowest amount of energy and have the smallest carbon footprint of all the technologies in the table. It is hard to beat a SSFCW in life cycle cost.
Numbers are also given for lagoon systems and FBRs and these are compared to conventional technologies.
Energy and Carbon requirements for various WWT systems
Lessons
