Ben Hazard of Te-Tech Process Solutions discusses how primary sludge fermentation can improve biological P removal and reduce chemical usage, sludge production and operational costs.
With tightening phosphorus consents and population growth, AMP7 has seen an increase in phosphorus removal schemes across the industry and this number will grow even more in AMP8. The default solution for many smaller works is to install chemical dosing and tertiary treatment, as this appears superficially to be the simplest solution with the lowest capital cost. However, the main chemicals used for P precipitation (iron and aluminium salts) are not a sustainable resource and, with supply chain issues, demand is expected to exceed the supply in the not-too-distant future, affecting price, quality and reliability of supply. It’s time for another look at enhanced biological phosphorus removal technologies.
Primary Sludge Fermentation Enhances Bio-P Removal
Primary sludge fermentation (PSF) has the potential to significantly enhance the phosphorus removal performance by enabling Enhanced Bio-P Removal (EBPR). By increasing the volume of volatile fatty acids (VFAs) available for phosphate accumulating organisms, biological phosphorus removal can occur where it was previously impossible due to insufficient VFA concentrations in the raw wastewater.
It’s Sustainable & Cost Effective
Alongside improved EBPR, PSF reduces the need for chemical precipitants and can potentially remove the need for chemical dosing altogether, bringing both cost savings and improving resilience and sustainability. Additionally, PSF reduces the volume of excess sludge produced, reducing sludge disposal and treatment costs.
All about VFAs
Whilst the biochemistry is complex, the principles are quite simple. In anaerobic conditions bacteria called “phosphate accumulating organisms” (PAOs) consume VFAs and release phosphorus as phosphate. Then in aerobic conditions they take up phosphate and convert into polyphosphate chains within their cellular structure. Given the right conditions and with sufficient VFAs available, the uptake of phosphorus in the aerobic stage is greater than the release of phosphorus in the anaerobic stage, so there is net reduction on phosphorus which is removed with the biological sludge in the final settling tank after the aerobic stage. Properly managed, this removal can exceed 80%. It’s mostly about getting the right amount of VFAs.
PSF ensures that there is a sufficient quantity of VFAs, by fermenting some of the primary sludge anaerobically for several days, which increases the VFA concentration. After settlement, the VFA rich supernatant is returned to the anaerobic “selector tank”.
The Process of PSF
The process consists of three separate stages:
1. Primary sludge is directed to one of two alternating batch fermenters which operate out of phase with each other to give a pseudo-continuous flow through the system.
2. The fermented sludge is mixed with raw wastewater in an elutriation tank for enhanced washout of the VFAs and pH regulation.
3. Finally, the fermented primary sludge and wastewater mix is separated in a thickener with the VFA-rich supernatant being sent to the anaerobic tank of the biological treatment stage.
To achieve optimal VFA production rates, the parameter that needs to be controlled is the sludge retention time in the PSF reactor tanks. This is quite an ambitious task as the VFA production rate depends on a number of different variables such as the reactor temperature, certain sludge specifics (e.g. origin of volatile suspended solids), and the presence of inhibitors. Online temperature, pH and ORP monitoring indicate where the process lies in terms of hydrolysis and subsequent fermentation. Based on this, the control system creates an optimal operational regime for sludge feeding and withdrawal, producing around 100 – 200 g VFA/kg COD.
In conventional activated sludge plants, a significant portion of COD is removed from the main process line in the primary sludge. However, primary sludge fermentation returns about 30% of the COD removed to the main process line with the fermenter effluent, although it is typically readily degradable.
Packaged PSF Systems
Te-Tech Process Solutions have developed the te-fermTM packaged primary sludge fermentation system that can be easily retrofitted even into small plants. It uses conventional technology together with real-time control based on online monitoring of temperature, pH and oxidation reduction potential (ORP) to maximise the rate of VFA production.
A complete process evaluation service for companies considering te-fermTM including a full evaluation of cost benefits and sustainability is available from Te-Tech.
Contact Te-Tech now: enquiries@te-tech.co.uk
