One of the goals for the UN decade for sanitation is to half the proportion of people lacking access to sanitation by 2015. Many countries will thus increase implementing or enhancing wastewater treatment and, considering the cost, it becomes very relevant to select appropriate technologies to limit resources needed for construction, and operation and maintenance (O&M) of the facility. The upflow anaerobic sludge blanket (UASB) technology is increasingly being applied for municipal wastewater treatment applications given its low-cost and simple operations.
UASB reactors were developed in the 1970s for the treatment of highly concentrated industrial wastewater. Following the initial reactor designs for the treatment of sugar industry wastes, the benefits of the system, such as low sludge production, small footprint, low energy requirements and valuable biogas production, made the UASB reactors an attractive and hence widely applicable treatment alternative for highly concentrated wastewaters at mesophilic temperatures.
These advantages encouraged investigating the application of the UASB process to the treatment of domestic wastewater from municipal applications, where the low BOD concentrations, coupled with high-particulate BOD fractions, results in insufficient methane production for heating the reactor to mesophilic temperatures. Since even moderate wastewater temperatures favour satisfactory removal rates within a reasonably sized reactor, the use of UASB reactors for domestic wastewater is becoming widespread in tropical countries such as Brazil, Colombia and India. Although this technology can not by itself produce an advanced effluent of the quality of a conventional secondary process like activated sludge, it can achieve 60-75 % BOD5 removal rates at a fraction of the construction and O&M cost.
In applications requiring higher treatment levels, UASB
reactors are usually followed by a polishing step, even though limited experience is currently available regarding polishing options for UASB effluent for larger size applications. Yet, UASB technology, followed by polishing steps, is a treatment option increasingly implemented in warm countries, with minimum wastewater temperatures over 18°C.
More recent tests have shown the benefits of low solids production and low energy consumption can also be achieved in more moderate climates, with hydraulic retention times only slightly higher and treatment efficiency only slightly lower when the temperature is reduced. Low-cost and low-tech solutions are often more appropriate than highly mechanised and complex conventional solutions, such as activated sludge – which uses a lot of energy to produce a high amount of biosolids, which in turn needs to be stabilised and disposed of at high cost.
With total plant construction costs similar to a primary treatment plant (including sludge handling facilities), an UASB-based plant is simpler, less reliant on mechanical components and can achieve on average double the organic matter removal rates of conventional primary treatment. In addition, the UASB process will generate substantially lower quantities of sludge, having a yield of around 0.3kg TSS/kg BOD. The generated biogas could be used to drive the aerobic downstream processes, leading to energy-autonomous treatment plants. Nevertheless, given the anaerobic nature of the process, a high potential for H2S generation (thus odour and corrosion problems) exists for UASB municipal applications with significant sulfate contents in the wastewater. Municipal UASB installations requiring relatively high treatment levels have traditionally relied on simple polishing technologies such as facultative lagoons.
This combination has limited applicability in larger installations because it becomes increasingly difficult to satisfy the space requirements of the lagoons. Under these circumstances, more compact polishing steps, such as activated sludge or biofilm reactors, are being considered.
Bucaramanga WwTW, operating since 1991, is one of the oldest large-scale UASB-based facilities treating municipal wastewater in the world. The plant’s design flow is 740 l/s based on the operation of four UASB reactors and serves an estimated 240,000PE. Currently, the plant is operating three reactors at approximately 540 l/s. It is anticipated the fourth reactor will be placed in service at the end of 2004.
The UASB reactors are covered with aluminum and the collected biogas is flared. Pre-treatment for the UASB reactors is provided by 6mm screens, followed by grit channels. The reactors are followed by two facultative lagoons. Construction costs for the existing facilities were approximately 20£/PE. Operating costs are in the order of 0.015£/m3 treated. Even though this plant has been able to achieved essentially secondary treatment levels at a very low capital and O&M costs, its biggest challenge has been H2S – related equipment and facility corrosion and the odour complaints from the surrounding community.
An odour control study identified ambient H2S concentrations greater than 500ppm in the vicinity of the reactors and of 12ppm in the facultative lagoons. The potable water in Bucaramanga has elevated sulfate levels and most of the sulfates present in the raw wastewater are effectively reduced to H2S in the anaerobic reactors. Addition of aluminum covers and the collection and flaring of the biogas has helped reduce the intensity of the odours and the corrosion of the equipment.
A large ongoing sewage infrastructure project (ParanaSAN) incorporates new collection system components, expansion and upgrade of five existing treatment facilities, as well as the construction of 18 new treatment plants for the city of Curitiba in Brazil and surrounding areas. One of the key features of the adopted wastewater treatment scheme is the inclusion of anaerobic processes in the liquid treatment train for municipal wastewaters. All of the new treatment facilities under ParanaSAN will incorporate this low-cost and simple technology as a first step.
The UASB reactors, which can remove 60-75% of the influent BOD5 load on an average basis, will be followed by a polishing step to further remove organic matter. Under the ParanaSAN project, a technical economical evaluation of viable polishing options was conducted for treatment facilities ranging in capacity from 0.5-1m3/s (10-24mgd). Polishing processes considered included unaerated lagoons, trickling filters, biological aerated filters and dissolved air flotation.
Table 1 presents a summary of the results of the evaluation of UASB effluent polishing alternatives. The information is presented in terms of unit cost estimates to facilitate its application elsewhere. The observed variations in the unit cost ranges take into consideration plant specific issues such as site conditions and industrial discharge contributions to the municipal wastewater flows.
From these numbers it can be observed that significant construction cost differences exist between polishing alternatives. Lagoon-based systems and DAF technology are the lowest cost polishing options, not even considering the much lower energy needs and solids disposal. Even when added to the cost of a base UASB facility, the 20-30£/PE of BAF compares well against the 50-100£/PE typically associated with conventional aerobic secondary treatment systems such as activated sludge.
ajman wwtw, united arab emirates
UASB reactors will be the first biological stage of treatment for a new treatment plant being built in Ajman, one of the United Arab Emirates. It is anticipated this plant will be operational in 2005. The aim of the Ajman project is to provide a centralised wastewater collection and treatment system in Ajman, collecting and treating the entire Emirates’ wastewater and produce a treated effluent for use in irrigation. Since there is presently no collection system or an existing treatment plant, design flows were calculated based on PE for 2015, which is the initial phase of the project, and for 2030, the final phase of the project.
Average flow rates were calculated based on 175 l/d/capita, assuming 50% of the consumed water will be returned via the sewerage system, as has been observed in neighbouring Emirates. The infiltration rate into the treatment plant was assumed to be 10%. The plant has been designed to treat a peak flow of about 1.6 times greater than the average flow. The pollution load that will be entering the plant was calculated based on a value of 60g/d/capita for both BOD and TSS.
Following pre-treatment of the influent in the form of screening and grit removal, the treatment plant has been designed as a two-stage biological process with UASB reactors and submerged aerated filters (SAFs) as the two biological treatment processes. Deep bed sand filters will be used after the biological treatment stages to ensure compliance with the strict effluent requirement of 10mg/l BOD5 and 10 mg/l TSS. As the effluent is to be used as irrigation water, the final stage in the treatment process stream is chlorine disinfection with a requirement of 200fecal coliforms/100 ml. Sludge processing consists of centrifugation of sludge wasted from the UASB reactors. A simplified process schematic of the facility is shown in Figure 1.
The UASB reactors for the Ajman WwTW have been designed based on the hydraulic design criteria compiled from experience in Brazil and from the Rio Frio WwTW discussed before. To enable the construction and optimise the operation of the future works, the plant has been designed with an anaerobic treatment stage composed of eight reactors, with a square geometry, using a 23.5m side/length and 5m liquid depth. Based on operational experience in Colombia and Brazil, the BOD5 removal rate for the UASB reactors in the Ajman project is assumed as 70% and the TSS removal is assumed as 75%. The downstream SAF and DBF are designed accordingly, allowing the storage of the aerobic solids in the deep bed filter for occasional return to the UASB for stabilisation. Overall solids yield is estimated to be 0.4kg TSS/kg BOD, compared to a yield of 1kg TSS/lg BOD for conventional primary and secondary treatment