Introduction to Algal Based Wastewater Treatment
The single largest expenditure in modern wastewater treatment is the energy required for aeration - supplying the oxygen required for the aerobic biological system. The new system developed by Aquanos utilizes oxygen produced by microalgal photosynthesis, and therefore dramatically reduces the energy requirements and operational expenditure, creating a system which is significantly more cost effective both in terms of CAPEX and in terms of OPEX, when compared to conventional systems.
Moreover, since anaerobic digestion is employed utilizing excess algae and biomass, biogas is produced, which can then be used for heat or power generation. Offgas from the combustion process, rich in CO2, is then reintroduced into the system, to promote intensive algal growth and maximize oxygen production. This feature has the added advantage of CO2 sequestration, which has both environmental and commercial advantages.
The Aquanos system can therefore be applied to both retrofit extensive WWTPs, while offering the above-mentioned advantages, or to serve as an extremely cost-effective alternative for applications in areas where infrastructure may not be sufficiently developed to supply the constant power required for high quality wastewater treatment.
Our process utilizes the symbiotic relationship between algae and microorganisms in Wastewater treatment. Algae produce oxygen through photosynthesis and use the nutrients present in the wastewater while the bacteria utilize the oxygen to break down organic and nitrogenous compounds, and produce CO2 which is then taken up by the algae as a carbon source.
This process is well known and has been used in extensive treatment processes (mainly lagoons) for many years. Nevertheless, while having a symbiotic relationships, high concentrations of microorganisms (such as is found in conventional wastewater treatment plants) will prevent light from penetrating into the water and will thus hinder algae growth. As a result, in traditional algae based wastewater treatment systems, a low concentration of biomass is retained and long retention times (20-70 days) are required, resulting in very large footprints. In addition, consistently producing high quality effluent (mainly in terms of nutrient removal) is hard to guarantee using lagoon systems.
The new Aquanos system overcomes these obstacles by dividing the process into 2 separate reactors.
1. Fixed film biological treatment – in this stage of the biological treatment, the microorganisms are attached to media.
2. Algae growth area (Raceway) – In these shallow, engineered algae ponds, the algae utilize nutrients and CO2 (generated by the microorganisms) to grow while producing water that is super-saturated with photosynthetically produced oxygen in higherconcentration.
The end result is that the microorganisms receive two streams, one of raw wastewater and the other, of oxygenated water from the algae pond. This allows for high quality treatment in 24 hours retention time.
Wastewater is screened and degritted before being introduced into an anaerobic stage. Depending on the application, this may be anything from a simple lined earthen anaerobic lagoon to a fully engineered Upflow Anaerobic Sludge Blanket (UASB) reactor. This stage is meant to reduce organic loading, as well as serve as a sink for excess algal and bacterial biomass and potentially produce biogas.
Following the anaerobic stage, the wastewater is introduced into a Moving Bed Biofilm Reactor (MBBR) where aerobic COD removal and nitrification take place. In the process, the active biomass is grown as a biofilm on small, cylindrical HDPE elements of about 10 mm X 10 mm, designed to maximize protected surface area and mass transfer of substrates and oxygen from the bulk liquid into the biofilm. Unlike activated sludge, the biomass carriers (with the active biomass) are retained within the reactor by sieves, and only excess biomass, shorn off the biomass carriers, is carried over to the final solids-separation system (clarifier, dissolved air flotation unit, or filter) with the effluent. The use of an attached growth system, where the biomass is confined physically to one unit and does not flow through the system, allows the oxygen-rich algal liquid to flow through the system with minimal mixing of algae and bacterial mass; this in turn minimizes contamination of the algal raceway with bacterial biomass, flocs, etc, thereby improving environmental conditions for the algae.
Oxygen to the MBBR is supplied by algae-rich water residing in open raceway reactors. The area, number and staging of the raceway reactors will depend on the process characteristics and amount of oxygen required. The oxygen-rich liquid is recirculated to the MBBR utilizing high-flow, low head pumps; from the MBBR the now oxygen-depleted liquid is returned to the raceway to resume photosynthetic oxygen production.
Final effluent is conveyed either from the MBBR or from the raceway to a final solids-separation unit; depending on the type of application and required effluent quality, this could be anything from a simple clarifier or lagoon to a Dissolved Air Flotation unit. Excess biomass (algal and bacterial) is removed from this stage for further processing, either in the digester, or for production of fertilizer, animal feed, or, in the future, biofuel.
The anaerobic and aerobic segments are sized according to standard design procedures for UASB (or anaerobic lagoons) and MBBR systems, respectively. Raceway ponds with hydraulic retention times of 24 hours were found to be sufficient for full BOD removal.
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