Calculation of hydraulic load for UASB

1. Water inflow rate/cross-sectional area; 2. Volume. It’s difficult to answer for items 3 and 4 – pilot testing is necessary! The structure of the reactor, the strain used, and the properties of the wastewater determine whether granular sludge can form, as well as the proportion of granular sludge in the total sludge. For example, in pulp and paper wastewater, it is difficult to increase the proportion of granular sludge in UASB.

1. Does it mean that “hydraulic load = inflow volume/area”, and “upward flow velocity = (inflow volume + circulation volume)/area”? Is that correct? 2. In terms of units, hydraulic load and upward flow velocity are essentially the same; it’s just that different numerators are used in the calculations. Is this understanding correct? Thank you for the advice!

I have been operating an anaerobic fluidized bed for five years now, and there are still significant differences between actual operation and the design! No matter how precise the design and construction are, various problems will still arise during actual operation. Take a simple example – suppose the designed HRT is 12 hours, but the conditions of water supply change; if the wastewater generation unit tries to increase production or shuts down due to a malfunction, the residence time will inevitably be shortened or extended! I don’t know if you work in design or operations; if you’re in design, it’s understandable to strive for perfection and thoroughness. If it’s about operation, there’s no need to focus so much on these parameters, as the treatment facilities have already been built and many of these parameters cannot be changed. It’s better to focus on improving the treatment efficiency by adjusting basic factors such as temperature, pH value, water distribution intensity and uniformity, as well as the sludge condition (the proportion of granular sludge, sedimentability, gas production rate), etc. What do you think?

The UASB consists of three parts: a sludge reaction zone, a gas-liquid-solid three-phase separator (including a sedimentation zone), and a gas chamber. A large amount of anaerobic sludge remains in the lower reaction zone, and the sludge with good sedimentation and coagulation properties forms a sludge layer at the bottom. The wastewater to be treated flows in from the bottom of the anaerobic sludge bed, where it comes into mixed contact with the sludge in that layer; the microorganisms in the sludge break down the organic substances in the wastewater, converting them into biogas. Biogas is released in the form of tiny bubbles. As these bubbles rise, they merge with each other to form larger bubbles. At the upper part of the sludge bed, the agitation caused by the biogas results in a sludge mixture with a lower concentration of sludge, which rises together with water into the three-phase separator. When the biogas encounters the reflective plate at the bottom of the separator, it is deflected around that plate and then passes through the water layer to enter the gas chamber, where it accumulates. The biogas is then extracted using pipes. The solid-liquid mixture is directed back into the sedimentation area of the three-phase separator, where the sludge in the wastewater flocs together, the particles grow in size, and they settle due to gravity. The sludge that settles on the inclined wall slides back into the anaerobic reaction zone along that wall, resulting in an accumulation of large amounts of sludge within the reaction zone. The treated water separated from the sludge overflows from above the overflow weir in the sedimentation area and is then discharged from the sludge bed. It is generally believed that in anaerobic reactors, an appropriate carbon:nitrogen:phosphorus concentration ratio available for microbial use is (200–300):5:1 (where carbon is expressed as COD, while nitrogen and phosphorus are based on their elemental content). Increasing the nitrogen level slightly at the start-up stage helps promote microbial growth and enhances the reactor’s buffering capacity. When the wastewater to be treated lacks these nutrients or their ratios are not optimal, it is possible to achieve the desired levels and ratios by adding them artificially; for example, high-concentration domestic wastewater can be added to supply carbon sources, urea can be used to supply nitrogen, and KH2PO4 can be used to supply phosphorus. (2) The hydraulic retention time has a significant impact on the upward flow velocity within a USB reactor. On one hand, when the hydraulic retention time is short, the upward flow velocity within the USB reactor is higher, which facilitates better contact between sludge and wastewater, improves mass transfer, and promotes microbial growth. It also aids in the separation of granular sludge from floculent sludge, accelerating the formation of granular sludge. On the other hand, to prevent excessive loss of sludge within the reactor, the upward flow velocity should not be too high; in other words, the hydraulic retention time should not be too short. Generally, the upward flow velocity in the reaction zone should be no less than 0.5 m/h, as this is one of the key conditions for the formation of granular sludge. When using anaerobic processes to treat low-concentration organic wastewater, the hydraulic retention time may be an even more important control parameter than the organic load rate. (3) In USB reactors, the hydraulic upward flow velocity is usually between 0.4 and 1 m/h. The sludge bed appears to be a stationary bed. In contrast, expansion granular sludge bed reactors (EGSBRs), which have a structure similar to that of USB reactors, use a larger height-to-diameter ratio and effluent recirculation, resulting in an upward flow velocity of 5–10 m/h; thus, the entire sludge bed is in an expanded state. Expanded granular sludge has a higher mass transfer efficiency with the organic matter present in the wastewater. Additionally, the higher upward flow velocity helps to enhance the hydraulic screening effect on the sludge, allowing floculent sludge to be removed from the reactor, which is beneficial for the growth of granular sludge. (4) Initially, a sludge load rate of around 0.1–0.2 kg COD/kg TSS·d is generally considered appropriate