LuminUltra – Waste Water
Biomass Inventory Management Using QG21W
The activated sludge process is used to concentrate the biomass which naturally degrades carbonaceous substances found in most industrial wastewaters. A common practice in these plants is to err on the high-side of biomass concentration to increase capacity against ‘slug loading’. Although this principle is sound in that additional biomass equates to additional capacity to degrade incoming wastewater, there are various downsides to this approach, including:
- Starvation – can promote ‘sluggish’ biomass, which perform at less than peak efficiency. This can occur when the plant has accumulated too large of a biomass population or if the plant experiences low flows and/or a drop in available food.
- Poor Settling – along with #1, high total solids inventory creates competition and results in advantages for filamentous microorganisms.
- Increased Waste Biomass – to offset new growth and maintain recycle rates, Waste Activated Sludge (WAS) flows must increase to accommodate higher solids inventory, meaning more waste biosolids requiring disposal.
- Poor Mass Transfer – increased solids concentration creates additional resistance to mass transfer for aeration, decreasing efficiency and increasing power requirements.
- Increased Pumping – increased solids loading creates additional wear on pumps and requires more dewatering, thereby increasing energy input requirements.
6. Aerobic Digestion – increasing solids concentration increases the possibility of aerobic digestion of old or dead biomass plus difficult-to-degrade wastewater constituents. Plus, digestion of old biomass consumes costly process additives such as oxygen and nutrients, and even results in the release of nutrients to the effluent. This also tends to happen when flow to a plant decreases, which translates to less food being available for the population to consume. In situations where this decreased flow is permanent, the treatment plant essentially becomes too big.
When operating at high sludge ages, a greater proportion of biomass activity is spent degrading the BOD from old, dying cells. This decreases operational efficiency since more return sludge is then needed to handle the treatment of wastewater BOD. Most treatment plants control sludge age by relying on general guidelines for maintaining Mixed Liquor Suspended Solids (MLSS) or Mixed Liquor Volatile Suspended Solids (MLVSS) targets. Unfortunately, these tests do not accurately reflect the viable biomass concentration in an activated sludge system and therefore are not reliable enough to fully optimize sludge return rate. MLSS cannot distinguish between biomass and other particulate matter, while MLVSS cannot distinguish between active biomass and non-cellular organic matter, unhealthy biomass, and dead biomass. The fundamental reason for this approach being a common practice is that the conventional indicator of biomass quality (Mixed Liquor Suspended Solids, or MLSS) is unable to distinguish between living, dead, or biologically-inert solids. With no way to be certain of the quality of living biomass, operators must maintain a MLSS to ensure sufficient treatment capacity.
LuminUltra’s Cellular ATP (cATP) parameter – an indicator of true living biomass quantity – provides the necessary peace of mind for process operators and engineers to reduce the quantity of total solids in their system and focus more on controlling and maintaining a stable living biomass population. Because cATP represents only the living portion of solids, it responds quickly and sensitively to process changes, especially the quantity of food entering the process. Thus, it provides a better normalizing parameter for calculating the important process F/M ratio. Measuring ATP also provides the earliest possible warning against process upsets.
This proposed practice is not new – more than 30 years ago, researchers from the University of Maryland (Levin et al., 1975) demonstrated the feasibility of reducing total solids inventory by relying on ATP measurements to improve treatment capacity in a municipal sewage treatment process. By reducing the return sludge flow over 40%, waste biosolids (i.e. WAS) production was reduced by 35% while BOD and TOC reduction across the plant improved.
Daily measurements of cATP can be used to quantify the living biomass in the process and confirm the presence of sufficient biomass to degrade the wastewater constituents entering the process each day. Purging the system of solids will allow living biomass to re-populate the process to create greater efficiency, resulting in sufficient cATP levels to degrade the incoming wastewater, plus a higher ABR and lower BSI. In addition, fewer particles in suspension results in greater mass transfer efficiency, reducing the power required to meet a specific DO set-point.
Benefits of Optimising Sludge Age
Electricity costs are one of the largest costs associated with wastewater treatment. In fact, the annual cost of aerating biological wastewaters in the United States is nearly $25 Billion US dollars. For example, the East Bay Municipal Utility District (Oakland, CA) reports electricity usage as outlined in the pie chart to the right (Water Environment & Technology, May 2004, P. 38). According to the figure, 66% of the average plant’s electricity bill is associated with aerating, mixing, pumping, or processing bioreactor solids.
Through optimizing and revitalizing your activated sludge treatment system, you can expect to save on electricity costs spent in oxygen delivery, sludge mixing, sludge pumping, and solids handling which are the major consumers in a typical plant. In addition to this substantial benefit, you can expect to also see improvements in the following areas:
- Waste sludge production is minimized – Experience has shown that ATP measurements can be used to optimize sludge age to run with less biomass and therefore less waste sludge, maintaining acceptable treatment efficiency, all while maintaining acceptable treatment efficiency (example: Environmental Science & Technology, October 1975, Pp. 961-965).
- Reduced Chemical Supplementation – Maintaining a smaller biomass population reduces not only oxygen demand, but also other chemical supplements (e.g. Nutrients) in industrial wastewater treatment plants.
- Increased Capacity – Lower sludge return increases the volumetric capacity of the reactor.
- Reduced Toxicity – The return of mature solids can cause toxicity to younger biomass due to the presence of dead organisms and concentrations of toxins.
Knowing the energy level and health of your bioreactor through ATP measurement, you can incrementally reduce the total solids inventory while maintaining the required amount of active biomass so that overall performance is not compromised. By incrementally returning less sludge, you not only permit the remaining population to grow faster, but you reduce the BOD loading on the system (because there is less BOD from the older microbes themselves). This means you need less power for aeration, because you are no longer treating the BOD of the bugs.
The graphs above provide an example of how cATP trending provides the ability to determine the optimum population for effective wastewater treatment and identify times when the reactor is overpopulated. The first graph shows two distinct periods of high and low active biomass populations. However, despite there being 51% less biomass in the second period, overall process performance was essentially the same in each period. The second graph indicates that there was indeed less BOD being fed to the process in the second period (14% less), but not nearly to the degree to which one would expect based on the population differences.
NOTE: Solids inventory optimization may require special care in certain situations (e.g. nitrification requires long sludge ages to enable nitrifying bacteria population to mature) due to specific sludge age requirements. For this reason, special care may be needed. Select an SRT suitable for the process.
The goal of this process is to reduce the total MLSS inventory while maintaining the same active biomass population. During this optimization process, it is imperative that MLSS and ATP data be collected on a daily basis for all reactor and RAS locations so that the necessary Food-to-Microorganism (F/M) and Solids Retention Time (SRT) calculations can be made to assess the optimization program.
- No two biological wastewater treatment processes are exactly alike, and therefore it is recommended that a baseline of data be established for processes that are new to ATP testing. To quickly generate sufficient data that can be used to generate control targets for stabilization and continuous process improvement, it is recommended that two or more bioreactor samples be measured daily for ATP for at least two weeks prior to undertaking the optimization program.
- Optimize the total solids inventory target over a period of days by using small step changes and observing the biological response.