Disinfection of Sewage, Stormwater and Final Effluent

This paper highlights the main disinfection options available for sewage, final effluents and stormwater, and is a review of the current research programme being carried out by staff of the Water Research Centre.
Disinfection is an increasing requirement, with particular emphasis on bathing and recreational water use. Laboratory and field trials are reviewed, and a wide range of disinfectants have been tested. These include peracetic acid and the Clariflow process for crude sewage; peracetic acid, ozone and UV irradiation for final effluent; and peracetic acid for stormwater.
The efficacy of the various disinfectants is evaluated and general costs are presented. Of special interest is the potential effect of sewage strength in reducing the disinfecting properties of the chemical. It is also apparent that there are wide variations in performance between processes and host organisms.     

Full-scale trials of recycled glass as tertiary filter medium for wastewater treatment

Pilot-scale trials at a domestic wastewater treatment works compared the performance of three grades of recycled glass (coarse, medium and fine) when used as tertiary filter media for total suspended solids removal (TSS). Fine glass produced the best effluent quality but blinded rapidly and coarse glass could process three times the flow but with a reduction in final effluent quality. The medium glass offered a compromise with similar flow characteristics to the coarse glass, yet still achieve good solids removal, albeit less than the fine glass. Full-scale studies compared the performance of medium glass with the sand medium that is typically used in this application. There was little difference between them in terms of TSS removal, and they both removed around 75% of TSS from the influent, provided that the solids concentration did not exceed 70mg/l. However, the glass media had superior flow characteristics and was able to treat an additional 8-10% of the influent following the backwash cycle. Over the study period, the influent to the filters had an average TSS concentration of 38mg/l and produced an effluent with an average of 15mg TSS/l. In order to design for an average TSS concentration of 20mg/l, the maximum solids loading on the medium should not exceed 0.25kg solids/m(3)/h. Selecting recycled glass as a tertiary filter medium will give a 10% reduction in the amount of media required, compared to sand. It also carries with it the benefits of using a recycled material, and although these are more difficult to quantify they include: reduced CO(2) emissions and use of a more sustainable product that promotes favourable publicity and positive environmental reporting.     

New media for effluent filtration

Although deep bed filtration is not new in industrial effluent treatment and advanced sewage treatment, new filter media have been investigated with a view to improving the process. The usual media, like sand and anthracite, may be augmented or replaced by hydroanthracite, pumice and expanded slate. The properties of these media are described. Laboratory and pilot tests illustrate their use for effluent treatment in Switzerland.     

Construction and Operation of a Submerged Aerated Filter Sewage-Treatment Works

The capacity of the sewage-treatment works at Silchester in Hampshire has been enhanced and the effluent quality improved to a high standard by the installation of a new submerged biological aerated filter plant. Ten prefabricated steel submerged aerated filters reduce BOD and fully nitrify settled domestic sewage from a population equivalent of 20 000.
Comprehensive operating data are presented to demonstrate the ability of single-stage submerged downflow aerated biological filters to produce a nitrified effluent. The final effluent quality remains consistently within consent limits at 4.8 mg/l BOD, 7.0 mg/l SS and 0.5 mg/l NH₃-N, all 95 percentiles. (Consent 7 mg/l BOD, 25 mg/l SS, and 5 mg/l NH₃-N, 95 percentiles.)
Data are presented on total coliform and E. Coli concentrations through the filter plant, showing better than 3 log reductions in the effluent.     

炭砂滤池在饮用水处理中的研究现状及前景

面对普遍存在的微污染水源水质和生活饮用水卫生标准的日益提高,自来水厂有必要采用深度处理工艺。炭砂滤池在保证有效去除浊度的基础上,可以增强对有机物和氨氮等污染物的去除效果,且炭砂滤池只需对水厂砂滤池进行改造,基建及日常运行管理费用较低,因此适用于我国自来水厂的提标改造。炭砂滤池的运行受到反冲洗、温度、预氧化和空床接触时间等因素的影响,对于其出水安全性尤其是出水生物安全性的问题亟待研究。为保证炭砂滤池在工程上的成功应用和推广,需进一步研究滤料级配、组合工艺运行以及滤池内的微生物特性。     

The Production of High-Quality Effluents in Sewage Treatment Using the Biocarbone Process

The Biocarbone process is a new system for water and effluent purification in which biological treatment and physical filtration take place in the same reactor. Because the use of the process for tertiary treatment of sewage effluent had not been fully investigated, a pilot-scale evaluation was undertaken at Severn Trent's Wigston sewage-treatment works. This paper presents the results.
The study corroborates performance information published by the manufacturer. It shows that the process is an alternative to conventional technology for producing fully-nitrified tertiary effluents having low biochemical oxygen demand (BOD) and suspended solids (SS) concentrations.     

Sloughing of microbial film in trickling filters

A well established model for microbial film growth in trickling filters has been used to study the influence of sloughing on the performance characteristics of trickling filters under conditions of constant input. It has been argued that sloughing occurs when the substrate concentration at the support-microbial film interface falls below a critical value. Under these conditions the effluent BOD fluctuates in a fashion similar to that found in filters treating sewage. This is consequence of the time dependent variation of the microbial film thickness throughout the filter.The results obtained lead to suggestions for improved performance and indicate that without proper understanding of the sloughing process, optimal design and operation of trickling filters is unlikely. It also appears that design procedures based upon an average microbial hold-up are inadequate.     

Corrosion control enhancement from a dolomite-amended slow sand filter

The associated decrease of pH in slow sand filters, due to CO2 conversion and biological activity, may produce effluent that is slightly corrosive to downstream distribution pipe material. This pilot study examined the use of a 3-cm crushed dolomite limestone media layer placed within the filter column of a slow sand filter to enhance effluent corrosion control by the introduction of beneficial dolomite dissolution products, without impacting turbidity removal efficiencies. Turbidity removal, calcium concentration, pH, conductivity, total hardness and alkalinity changes were calculated for the filter during a 60-day pilot study, and water chemistry values were used to estimate the changes of the saturation index (SI) throughout the filter run. Total hardness change through the filter was compared to change calculated by a derived equation for hardness using calcium concentrations to determine if the media was dissolving in stoichiometric proportions, and mineral service life in the filter was estimated using an assumption of stoichiometric dissolution at a constant flow rate. Effluent SI was raised an average of 30%, alkalinity was increased by 19%, and effluent pH averaged 7.7. Filter effluent complied with current turbidity regulatory requirements for the provision of potable water, and mineral service life was estimated between 7.5 and 9.5 years.     

Commissioning and Operation of Yau Kom Tau Water-Treatment Works (Hong Kong) Using Direct Filtration

The paper outlines the commissioning, performance trials, and operational experience of the direct-filtration process at the Yau Kom Tau treatment works in Hong Kong.
The commissioning tests established the capability of the treatment plant to produce water of the required potable quality. However, the contractual performance trials were carried out at a time when adverse raw water might be expected, in order to assess the overall attainment of guarantees on the treated water quality, filter run, and washwater percentage.
Since the works became operational in November 1985, a number of incidents of raw water quality outside the design limits have been experienced. The use of polyelectrolyte was found to enhance the performance and reliability of the direct-filtration process.
The paper also reports the results of some optimization tests and comments on the overall performance of the plant compared with expectations at the time of design.     

Operation of Submerged Filters by Anglian Water Services Ltd

Since the introduction of submerged filters for sewage treatment in Europe, process improvements have been rapid. Downflow submerged filters using natural medium have been superseded by upflow filters using synthetic floating medium. Many advantages have been claimed for these systems and Anglian Water have investigated the operation of the Biobed and Filtabed submerged filters for carbonaceous oxidation, nitrification and suspended-solids removal. The process is based on the use of a bed of floating plastic beads (2–5 mm) through which the sewage flows in an upward direction. The reactor is aerated or non-aerated according to the required application. Results have shown the process to be capable of producing an effluent containing less than 2 mg/l amm.N and 5 mg/l SS. Backwash water requirements are less than 1%. The Biobead/Filtabead systems are designed to be competitive with regard to area, capital and operating costs.     

Monitoring the Effect of Sewage Effluent on the Oxford Canal Using Chironomid Pupal Exuviae

Collections of chironomid pupal exuviae from the Oxford Canal were taken monthly from May to October 1991 at stations above and below the sewage effluent discharge at Kidlington, near Oxford.
The chironomid exuvial assemblages showed clear changes reflecting the effects of the sewage, notably in the increased dominance of Parachironomus arcuatus and Chironomus riparius and other pollution-tolerant species downstream from the effluent inflow. A return to the pattern of taxa seen in the upstream stations was noted below the next canal lock, 2.8 km downstream from the effluent discharge.
Environmental quality indices, calculated according to the Chironomid Pupal Exuvial Technique, also reflected the effects of the sewage inflow.
The results show that chironomid populations, as sampled by collecting pupal exuviae, are potentially valuable for monitoring organic pollution in canals.     

BAF反冲洗污泥的特性及絮凝效能

曝气生物滤池(BAF)反冲洗污泥的处理是影响其工程应用的技术难题之一，为此考察了BAF反冲洗污泥的沉降特性及絮凝性能，系统分析了其在生物一化学强化混凝处理生活污水时的效能及二次污泥的特性。研究结果表明，BAF反冲洗污泥具有较强的生物活性和絮凝活性，将其与金属盐混凝剂组合预处理生活污水时出水水质满足曝气生物滤池的进水水质要求，而且处理过程中产生的二次污泥具有较好的沉降及脱水性能。     

The effects of media size on the performance of biological aerated filters

Biological aerated filters (BAFs) are an attractive process option, particularly when low land usage is required. They can combine BOD, solids and ammoniacal nitrogen removal and can be utilised at both secondary and tertiary stages of wastewater treatment. Media selection is critical in the design and operation of BAFs to achieve effluent quality requirements. Two size ranges, 1.5-3.5 and 2.5-4.5 mm, of a foamed clay called StarLight C were used in pilot-scale reactors. Both performed well as BAF media, with reactor loads up to 12 kg COD m(-3) d and 4 kg suspended solids m(-3) d (based on working volumes). The most consistent effluent was obtained using the smaller medium since, at flow rates above 0.41 min(-1), the BAF using the larger medium produced an effluent containing more than 20 mg l(-1) of suspended solids for over 30 min after backwashing. Up to 70% longer run times, as determined by reaching a set head loss, were recorded for the BAF containing the larger rather than the smaller medium. Additionally, the development of pressure above the smaller medium filter bed tended to be logarithmic rather than linear. Reactor profiles indicated that suspended solids removal did not occur over the full 2.3 m depth of the columns. The BAF containing the smaller medium utilised a mean depth of 1.7 +/- 0.3 m, whereas a mean depth of 2.1 +/- 0.3 m was used by the larger medium BAF. Both the head loss development data and the suspended solids removal profiles indicated that the smaller medium BAF was underperforming as a filter.     

城市污水曝气过滤式化学除磷试验研究

磷污染与水体富营养化密切相关，研究开发经济、有效的除磷方法是解决当前水体富营养化问题的迫切需要。为此开展了曝气过滤式化学除磷的试验研究，考察了富铁填料层厚度、滤速、气量等对除磷效果的影响。结果表明，曝气过程和曝气量对除磷效果有重要影响，通过适量曝气既可以提高除磷率，又可以控制出水铁含量；当滤速≤3m／h时，滤层厚度〉0．4m便可使出水总磷达到排放标准；气水联合冲洗是对多孔富铁填料进行再生的有效方法。     

Comparison of Algal Penetration Through Rapid-Gravity Filter Beds

T ests OF FOUR types of filter media are presented, which show that granular activated carbon performs marginally less well than anthracite/sand or anthracite/sand/garnet in the removal of algae, particulate organic carbon, iron and turbidity. The lengths of run which are achieved by the two granular activated carbon filters are also shorter than those of the other two media. A three-layer filter is better than the anthracite/sand filter for particulate organic carbon, iron and turbidity removal, and the filtrate contains lower mean concentrations of algae.     

Operational Experience with Biological Aerated Filters1

Yorkshire Water has two full-scale biological aerated filters in operation. The first was installed at North Bierley sewage-treatment works for the tertiary nitrification of humus―tank effluent to secure consent compliance and ascertain if anticipated future standards could be achieved. The second was recently commissioned at Hedon sewage-treatment works for secondary treatment, as part of an evaluation of compact plant to meet the requirements of the urban waste water treatment Directive at coastal sites. The plant at North Bierley (near Bradford) is constructed mainly of steel and is designed to achieve an effluent quality of 10 mg/l suspended solids, 10 mg/l BOD, and 5 mg/l ammoniacal nitrogen. The plant at Hedon (near Hull) is constructed of concrete and is designed to achieve a 30 mg/l suspended solids and 25 mg/l BOD standard. Both plants have proved to be capable of meeting their respective effluent requirement. For each works, the paper describes (i) the identification of the problem, (ii) the reasons for selecting the biological aerated filter process, (iii) details of the plant, (iv) operating experiences, and (v) plant performance.     

Specific deposit and models of horizontal counter-current filtration

Horizontal filtration with counter-current principle has got distinct advantages over conventional rapid sand filters (RSF), in terms of longer duration of filter run and better utilization of the entire media depth and overcomes most of the shortcomings of RSF, such as mud ball formation, development of negative pressure inside the filter media at relatively shallow operating depths of water over the filter media, etc. Even direct filtration may be feasible without secondary sedimentation. The present study was carried out in the laboratory on various models of horizontal filters with counter-current flow under varying discharge conditions with constant head, using sand and gravel as filter media and bentonite clay suspension in water as turbidity. In the coarsest media, incremental head losses were found to increase with time whereas in the finest media, it was found to be reversed. In the intermediate layers, incremental head losses increased at the initial filter run but decreased later on. The increase in specific deposit resulted in incremental head loss whereas decrease in permeability caused a decrease in approach velocity and hence decrease in incremental head loss. Coarsest media layer attracts maximum deposit and leads to more head loss increase as compared to other layers. Exponential decrease in effluent discharge in three distinct stages was observed during the filter run. Such a variation is modelled. Specific deposit is maximum in coarsest media but decreases sharply along the depth of media. Such a variation is also modelled. The depth of the media is increased in order to maintain throughout quality requirements. A model is presented to relate depth of media with duration for a discharge of more than 100 litres per minute per metre square (1 min⁻¹ m⁻²). It was observed that due to small magnitudes of specific deposit in the sand and gravel media, the exponential constants y and z in the Ives' model of efficiency are not important. The exponential constant x is however found to play a significant role. Methodology of evaluation of the exponential constant x in Ives' model is presented. This constant x is shown to remain around 1.7.     

Transport and fate of Cryptosporidium parvum oocysts in intermittent sand filters.

The transport potential of Cryptosporidium parvum (C. parvum) through intermittent, unsaturated, sand filters used for water and wastewater treatment was investigated using a duplicated, 2³ factorial design experiment performed in bench-scale, sand columns. Sixteen columns (dia=15 cm, L=60 cm) were dosed eight times daily for up to 61 days with 65,000 C. parvum oocysts per liter at 15°C. The effects of water quality, media grain size, and hydraulic loading rates were examined. Effluent samples were tested for pH, turbidity, and oocyst content. C. parvum effluent concentrations were determined by staining oocysts on polycarbonate filters and enumerating using epifluorescent microscopy. At completion, the columns were dismantled and sand samples were taken at discrete depths within the columns. These samples were washed in a surfactant solution and the oocysts were enumerated using immunomagnetic separation techniques.

The fine-grained sand columns (d₅₀=0.31 mm) effectively removed oocysts under the variety of conditions examined with low concentrations of oocysts infrequently detected in the effluent. Coarse-grained media columns (d₅₀=1.40 mm) yielded larger numbers of oocysts which were commonly observed in the effluent regardless of operating conditions. Factorial design analysis indicated that grain size was the variable which most affected the oocyst effluent concentrations in these intermittent filters. Loading rate had a significant effect when coarse-grained media was used and lesser effect with fine-grained media while the effect of feed composition was inconclusive. No correlations between turbidity, pH, and effluent oocyst concentrations were found. Pore-size calculations indicated that adequate space for oocyst transport existed in the filters. It was therefore concluded that processes other than physical straining mechanisms are mainly responsible for the removal of C. parvum oocysts from aqueous fluids in intermittent sand filters used under the conditions studied in this research.     

Development of a biological filtration model applied for advanced treatment of sewage

A mathematical model of biological filtration process is developed in this paper. A biological filtration process has advantages that filtration action and biological activities are combined in a single reactor with aid of filter media. Both physical and biological functions are incorporated in this developed model to simulate both mechanisms. Backwashing is expressed by the assumption that a mean captured solids concentration is input as data, and a captured solids concentration is kept at that value during each filtration run. The developed model is applied to explain the experimental performance with biological filtration reactors, in which batch cultivation of autotrophic bacteria and continuous treatment of actual sewage are carried out. Its applicability is discussed by comparing the simulated results with the experimental data. This model can favourably estimate maximum accumulation of autotrophic bacteria on the medium in batch cultivation, long-term treatment performance in continuous treatment, details of water quality profiles through the filter bed, and biomass. Required hydraulic retention time for nitrification and an appropriate recirculation ratio in a winter season are discussed with this model. This model predicts that a HRT of 1.1 h or above is required to achieve nitrification with remaining NH4(+)-N of less than 1 mgN/L and that an appropriate recirculation ratio is 2-3.     

An Evaluation fo the Clariflow Process for Sewage Treatment

T he performance of the Clariflow process has been evaluated at the Sandown sewage-treatment works of the Southern Water Authority, Isle of Wight, by the WRc Processes section. The process is designed for the partial treatment of sewage prior to discharge in estuarine or coastal locations. As a marine treatment it represents an alternative to the use of long sea-outfalls.
The Clariflow process effectively reduces total, faecal coliforms and salmonellae, and may reduce viruses. Screenings are totally removed and phosphorus and heavy metals are significantly reduced. Solids and BOD removal levels are superior to those achieved by conventional primary processes.
The paper reviews the development of the process, the results of the evaluation and cost information. Recommendations have been made to further optimize performance.