Treated as a valuable resource, municipal sludge, often today referred to as biosolids, is processed through avariety of novel unit operations leading to a safe, aesthetically pleasing, and sought-after product. The design engineeris concerned first with the ultimate disposal and utilization of the biosolids, providing at least two options for the finaldisposal. Volume reduction, stabilization or vector attraction reduction, and pathogen inactivation are the key goals; processtrains combining them into one unit process are the target technologies. Drying and pelletization are now being applied atmuch smaller plants because of the introduction of indirect dryers, which have fewer air pollution problems than the directdryers still used at some larger plants. Stabilization of biosolids in newer plants is more often combined with disinfectionat thermophillic temperatures, in anaerobic and particularly in aerobic regimes. For the smallest plants, dewatering is nowavailable in drying bags or vacuum drying beds, and larger plants benefit from an array of new devices offering sludgecakes as dry as 22 to 40% total solids. The ultimate dryness will depend on the quality of sludge, polymer conditioningprogram, and machine parameters. Emphasis on cost reduction, with simultaneous demand for an excellent quality endproduct, calls for innovative and case-specific solutions that go beyond the treatment plant and also address the quality ofindustrial–commercial discharges to the municipal sewers.
Comments
Content
75
Wastewater biosolids: an overview of processing, treatment, and management
J.A. Oleszkiewicz and D.S. Mavinic
Abstract: Treated as a valuable resource, municipal sludge, often today referred to as biosolids, is processed through a variety of novel unit operations leading to a safe, aesthetically pleasing, and sought-after product. The design engineer is concerned first with the ultimate disposal and utilization of the biosolids, providing at least two options for the final disposal. Volume reduction, stabilization or vector attraction reduction, and pathogen inactivation are the key goals; process trains combining them into one unit process are the target technologies. Drying and pelletization are now being applied at much smaller plants because of the introduction of indirect dryers, which have fewer air pollution problems than the direct dryers still used at some larger plants. Stabilization of biosolids in newer plants is more often combined with disinfection at thermophillic temperatures, in anaerobic and particularly in aerobic regimes. For the smallest plants, dewatering is now available in drying bags or vacuum drying beds, and larger plants benefit from an array of new devices offering sludge cakes as dry as 22 to ≥40% total solids. The ultimate dryness will depend on the quality of sludge, polymer conditioning program, and machine parameters. Emphasis on cost reduction, with simultaneous demand for an excellent quality end product, calls for innovative and case-specific solutions that go beyond the treatment plant and also address the quality of industrial–commercial discharges to the municipal sewers. Key words: sludge, biosolids, process design, dewatering, digestion. Résumé : Traitées comme une ressource précieuse, les boues municipales, désignées souvent aujourd’hui par le nom de biosolides, sont traitées par une variété d’unités d’opération novatrices menant à un produit sûr, satisfaisant esthétiquement et recherché. L’ingénieur de conception est concerné d’abord par la disposition et l’utilisation finale des biosolides, fournissant au moins deux options pour la disposition finale. La réduction de volume, la stabilisation ou la réduction du vecteur d’attraction et l’inactivation des pathogènes sont les buts principaux, alors que les processus de traitement les combinant dans une unité de procédés sont les technologies ciblées. Le séchage et la fabrication de pastilles sont maintenant appliqués à des usines beaucoup plus petites dues à l’introduction des sécheuses indirectes qui n’ont pas les problèmes de pollution atmosphérique des sécheuses directes, toujours utilisées dans quelques-unes des plus grandes usines. Dans des régimes anaérobics et en particulier dans des régimes aérobics, la stabilisation des biosolides dans les usines plus nouvelles est de préférence combinée avec une désinfection aux températures thermophiles. Pour les plus petites usines, l’assèchement est maintenant disponible avec des sacs de séchage et des lits de séchage sous vide, alors que de plus grandes usines tirent bénéfice d’un choix de nouveaux dispositifs offrant des gâteaux de boues aussi secs que 22 à ≥40% du total solide. La sécheresse finale dépend de la qualité de la boue, du programme de conditionnement des polymères et des paramètres de la machine. L’emphase sur la réduction des coûts, avec la demande simultanée d’un produit final d’excellente qualité, appelle à l’innovation et à des solutions spécifiques qui vont au-delà de l’usine de traitement et qui adressent également la qualité des décharges industrielles et commerciales aux égouts municipaux. Mots clés : boues, biosolides, conception de processus, assèchement, digestion. [Traduit par la Rédaction]
Introduction
Sludge generated during wastewater treatment has been traditionally regarded as a necessary nuisance by-product. There
are numerous examples of designers completing a wastewater treatment plant process with an arrow denoting “sludge to disposal.” Historically, many operators had to come up with innovative strategies for “disposal,” as storage lagoons were filled
in the first year of operation. Uncertainty as to the sludge quality (e.g., pathogen and metal content) acceptable for different disposal options, still existing in many provinces, further complicates the process. This perception is slowly changing, however. Wastewater sludge is being viewed as a valuable resource within the treatment process, providing volatile fatty acids for carbon-deficient systems, and the final product is perhaps one of the best sources of an agricultural soil conditioner and a source of slow-release nutrients and microelements. Regulations defining biosolids quality are slowly being developed in Canada and the European Union, with all countries capitalizing on the pioneering regulation developed in the United States. Among other requirements, the U.S. Environmental Protection Agency (USEPA) has delineated pathogen standards under the Resource Conservation Recovery Act (RCRA), Subtitle D, 40 CFR Part 503 pathogen requirement (USEPA 1994). These pathogen requirements are based on performance standards for two classes of pathogen reduction: class A (PFRP, a process to further reduce pathogens) and class B (PSRP, a process to significantly reduce pathogens). All sludges which are to be land applied must meet the requirements of one of the two classes. Class B biosolids include use and site restrictions, and class A biosolids have no restrictions on use. All sludges (biosolids) that are to be sold or given away must meet class A requirements. The process criteria for disinfection under rule 503 regulations are specific for each stabilization– disinfection process. Although there is significant criticism levelled at the USEPA from various sources, better regulations have not emerged from any other source to date. The objective of this presentation is to discuss the current state-of-the-art in sludge management from the standpoint of achieving the desired quality for final disposal. This paper concentrates on current and future trends, emphasizing technologies and practices that generate at least class B and, in some cases, class A biosolids. With a large land base, most municipalities in Canada will be opting for some type of land disposal. In the United States 63% of biosolids were land applied in 1998, with the number expected to reach 66% in 2005 and 70% in 2010. The authors expect similar trends in Canada.
Process choice and final disposal options
The array of unit processes available to today’s design engineer is overwhelming. Although many vendors claim superior process yields (e.g., biogas in anaerobic digestion with solids pre-treatment) or final total solids (TS) content in dewatering, there is very little in the literature that compares the processes side by side in large pilot or full-scale studies. Almost no costs comparisons are available, leaving the consultant to the processes he or she is most familiar with. The main difficulty is that, at every step of the sludge train, there are choices and the unit costs differ based on location and the size of the facility. Figure 1 attempts to present most of the processes available today (Oleszkiewicz 1999a). Even this extensive list is incomplete. The list should be treated more as the menu to choose
Oleszkiewicz and Mavinic Fig. 1. Some of the many unit processes available for sludge treatment (Oleszkiewicz 1999a).
77
advising caution in applying sludge (Harrison et al. 1999), there is also a more emotionally charged anti-sludge lobby such as the National Sludge Alliance in New York (Orlando 1997). To deal with such situations, a National Biosolids Partnership (NBP) has been formed in 1999 in the United States which combines under one umbrella environmental authorities, the agricultural community, an association of municipalities, sludge processors, and value-added product market developers. A nucleus ad hoc biosolids group, comprising various stakeholders, has been also been formed during Biosolids 2000, the 1st Canadian Biosolids Management Conference held in Toronto from 24 to 26 September 2000. In Canada, the regulators (such as the Canadian Council of Ministers of Environment or CCME) are moving more cautiously and are looking at the more stringent standards existing in Europe. The trends towards land application are, however, the same. In Toronto, for example, where some 70% of sludge produced was successfully incinerated, the plans are to have 100% land application, partly in the form of stable, wet solids and partly as dried pelletized material (Iamonaco 2000). A large and ongoing effort to convince the public about the benefits of land-applied biosolids is constantly needed. Figure 2 shows the process necessary in deciding on the choice of the treatment train and final disposal. The following points assist in interpreting Fig. 2:
• It may not be cost-effective to tolerate an offending industry that forces the municipality to abandon the disposal through land application. • Changing sociopolitical climate or ban on winter application may prevent disposal in wet form. A number of municipalities were suddenly forced to rethink their disposal options and look for an alternative disposal mode. • In jurisdictions lacking definitive sludge quality guidelines, permits may be negotiated based on the USEPA regulations (USEPA 1994).
Fig. 2. Diagram showing the process that must be used to decide on the choice of treatment train and final disposal. Choice of final disposal must precede the sludge train design process.
Fig. 3. Elements of a simplified solids mass balance in a biological nutrient removal (BNR) plant (Oleszkiewicz 1999b). BOD5, 5-day biochemical oxygen demand; PS, primary sludge; RAS, return activated sludge; TSS, total suspended solids; VFA, volatile fatty acids; WAS, waste activated sludge; Q, wastewater flow.
activated sludge leads to lower fractions of active biomass and a smaller nitrifier population. • Utilization of PS for production of short-chain volatile fatty acids (SCVFA). • Aerobic or mechanical thickening of WAS, with minimum residence time, to limit the release of nitrogen and phosphorus; dissolved air flotation (DAF) is often used in Canada.
of the work on sludge volume reduction focuses on the generation of WAS. The mass of WAS produced (expressed as volatile suspended solids, VSS) varies from 0.25 to ≥1.2 kg VSSproduced /kg BODremoved and depends on a number of factors, including the following (Oleszkiewicz 1998): (i) type of wastewater treated, particularly the ratio of degradable to nonbiodegradable fractions of chemical oxygen demand (COD), where an increased fraction of nonbiodegradable particulate COD increases the overall mass of sludge produced; (ii) inherent biomass yield Y (g VSSproduced /g BODremoved ), which depends on the nature of the biodegradable COD; (iii) solids residence time (SRT) in the system, where longer solids residence times result in less sludge being generated, as decay accounts for mineralization of organic matter; (iv) temperature, where a lower temperature leads to a larger mass of WAS being generated, as the decay (an internal metabolic process) slows down; and (v) type of process used, where processes involving anoxic and anaerobic zones with active reducing processes such as denitrification may produce less sludge per unit of removed BOD than a corresponding fully aerobic process (Jardin and Popel 1996), but unfortunately there are also data that show the opposite trend (Lishman et al. 2000). The data in Fig. 3 are approximate and are provided only for illustration of the quantities involved. The biomass yield was assumed to be Y = 0.6, based on empirical design data (WEF 1998). The nonbiodegradable part of total suspended solids (TSS) in the WAS was calculated to be 552 kg/d (Oleszkiewicz 1999b). A full solids balance requires several iterations and is presented in detail by Metcalf and Eddy (1991). It should also be noted that sludge quality depends on the type of wastewater treated and is particularly affected by the types of industry within the catchment area. Table 1 provides a partial characterization of “typical” raw sludges from domestic wastewater. Significant deviations from those numbers can be encountered in practice, particularly if industrial wastewaters are combined (Kiely 1997). The USEPA (1994) rule 503 limits for metals in Table 1 are for ceiling concentration limits (cc) and the “no adverse effects levels for land application” (NOAEL). The range of values actually encountered varies widely. The range for lead, for example, has been found to vary from 13 to 26 000 mg/kg TS. Drastic declines in metal contents have been observed in recent years in North America due to the implementation of tough sewer discharge bylaws (Bastian 1997).
Volume reduction: thickening
Sludge particles contain water in various forms. Currently, four different forms are defined and can be determined analytically (Kiely 1997): • Free, nonbonded water that can be removed easily by simple thickening, which increases the TS content from 1 to 4–8% TS. Some 75% of all water will be removed (approximately five times volume reduction achieved) with thickening to 4% TS.
80
J. Environ. Eng. Sci. Vol. 1, 2002
Table 1. Typical composition of untreated municipal wastewater sludge (Metcalf and Eddy 1991; Bastian 1997). Parameter Total solids, TS (%) Volatile solids, VS (% of TS) Protein (% of TS) Fats and grease (% of TS) Nitrogen (% of TS) Phosphorus (% of TS) Potash (% of TS) Alkalinity (mg/L as CaCO3) Organic acids (mg/L as acetic acid) Fecal coliforms (no./g TS) Salmonella sp. (no./g TS) Heavy metals (median, mg/kg TS)a Cd Pb Hg Ni Mo Sludge PS, 2–8; WAS, 0.6–1.2 PS, 60–80; WAS, 60–88 PS, 20–30; WAS, 32–41 PS, 6–30 PS, 1.5–4.0; WAS, 2.5–5.0 PS, 0.8–2.8; WAS, 1.5–11.0 PS, 0–1.0; WAS, 0.5–0.7 PS, 500–1500; WAS, 580–1100 PS, 2000 – 10000 MS, 106–107 MS, 5×102 24.86 170 2.33 70.6 24.93 Comments % TS in WAS depends on the ability of AS flocs to settle VS in WAS decreases with decrease in SRT in the biological system WAS protein decreases with decrease in SRT Decreases with tighter industrial and commercial sewer discharge % P in WAS depends on extent of biological P removal Depends on the extent of nitrification in AS Varies with the retention time in the hopper and temperature Class A must have <103 MPN/g TS, and class B <2 × 106 MPN/g TS Class A must have <3/4 g TS EPA rule 503: cc 85; NOAEL: 18 EPA rule 503: cc 840; NOAEL: 300 EPA rule 503: cc 57; NOAEL: 15 EPA rule 503: cc 420; NOAEL: 500 EPA rule 503: cc 75; NOAEL: 35
Note: AS, activated sludge; MS, mixed sludge; PS, primary sludge; SRT, solids residence time; WAS, waste activated sludge. aBased on a survey of 50 of the largest plants in the United States (Bastian 1997).
a typical 2.7% TS, and allowed for an increase in SRT to 21 d from the typical SRT of 13 d. This led to better stabilization and an increase in biogas yield from 197 to 211 L/kg volatile solids (VS) destroyed (Poling and Reynolds 1996). Thickening effectively increased the throughput of the digester, without increasing the hydraulic residence time (HRT). Recent application of the Torpey process in Phoenix, Arizona, led to savings of US$1 620 000, the equivalent cost of a new digester (WEF Biosolids 1999).
Fig. 4. Recuperative thickening (also called Torpey’s process) recycles active biomass and increases solids residence time (SRT) without increasing the size of the digester (Poling and Reynolds 1996). HRT, hydraulic residence time.
Vector attraction reduction: stabilization
The processes that decrease putrescibility, i.e., vector attraction reduction (VAR), include aerobic and anaerobic digestion, alkaline treatment, some heat treatment, and incineration. Some of the high-temperature processes may be preceded by biological stabilization to recover energy or assure the highest quality and durability of dried solids. The definition of “stabilized” solids is not uniformly accepted. The USEPA uses the value of 38% reduction of volatile solids as the threshold for considering the sludge stabilized, based on the work of Koers and Mavinic (1977). The current trend is to use processes which combine several unit operations in one stage. Thus, there has been a proliferation of thermophillic digestion processes and combination of processes (aerobic and anaerobic) which achieve stabilization, as well as disinfection. Thermal drying accomplishes essentially two operations, namely disinfection and moisture reduction. Although some equipment vendors claim that stabilization is also achieved during drying, most of the large generators of dried pelletized solids (Boston, Los Angeles, Toronto,
81 Fig. 5. Modification of the conventional mesophilic digestion aimed at increasing volatile solids (VS) destruction, gas production, and pathogen inactivation (Oleszkiewicz and Reimers 1999a): (A) thermophillic digestion with one or more reactors in series; (B) thermophillic aerobic pre-fermentation; (C) phasedtemperature digestion; (D) acidogenic pre-fermentation; (E) pulse power or ultrasound pre-treatment; (F) alkaline hydrolysis.
effectiveness) of biogas utilization becomes pronounced above 50 000 m3 /d. Since mesophilic digestion does not kill pathogens efficiently, some conventional configurations have recently been upgraded to thermophillic conditions (48–57◦ C); separate disinfection is also practised. Technically, the seven-stage microbial process leading to methane generation can be simplified to two stages, namely acid production from hydrolyzed organics and methane generation from acetic acid and hydrogen. The rates of each step are different and fast accumulation of end products of one may inhibit the other. For example, accumulation of hydrogen gas will inhibit acid-producing bacteria. Fortunately, hydrogen is an important energy source for the methanogens and is rapidly consumed, unless the hydrogen-utilizing methanogens are inhibited by some toxicant. Sulfide, produced by sulfate-reducing bacteria (SRB), when present in the un-ionized gaseous form is a typical inhibitor of methanogens. Ionized sulfides, dominating at more alkaline pH, are not usually a problem up to concentrations as high as 500–1000 mg S/L (Hilton and Oleszkiewicz 1988). The SRB utilize higher VFAs such as propionic and lactic acids, and some can actually utilize acetic acid, a substrate for methanogens. The presence of metals may lead to precipitation of sulfides. Given adequate time, the methanogens can acclimate to the presence of sulfides if the incoming sludge COD to S ratio is high enough (McCartney and Oleszkiewicz 1993). Table 2 presents the optimum operating conditions for an anaerobic digester. Close temperature control at about 36◦ C is necessary for the chosen temperature regime, as variations exceeding 2◦ C lead to temporary cessation of methanogenesis. The thermophillic reactors may be somewhat more difficult to operate and more vulnerable to ammonia toxicity. The relationship between the key design parameters, solids retention time (SRT) and volumetric loading, has been recently elucidated in a full-scale test in King County, Washington. For the tests with SRT equal to 14.9, 17.4, 34, and 81.4 d and corresponding loads (kg VS/(m3 ·d)) of 2.7, 2.4, 1.4, and 0.6, the respective destruction of VS was 52.2, 52.4, 54.5, and 57.2% (Butler et al. 1999). Clearly no benefit of a long SRT (larger tank) could be documented at mesophilic temperatures. Mixing and contact opportunity are paramount, hence the proliferation of the European, hydraulically more efficient (although more expensive to build) egg-shaped digesters (Brinkman and Voss 1999). Maintaining the parameters listed at other than optimum design levels will lead to less than optimum performance of the digester. In search of improved VS destruction, pathogen inactivation, and decrease of the required detention time, a variety of novel reactor configurations are being tried (Fig. 5). To bring the sludge to class A, usually a series of two reactors are used, such as in Fig. 5A, where two thermophillic reactors provide a barrier against the breakthrough of pathogens. Such a series arrangement is used in the new Annacis Island plant in Vancouver, British Columbia. A number of larger plants have upgraded to class A by introducing a very short HRT (typically under 1 d) thermophillic aerobic digester (TAD) (Fig. 5B). The TAD is op-
82 Table 2. Preferred operating conditions of a high-rate, single-stage, mesophilic digester. Parameter pH SRT (d) VS loading (kg/(m3 ·d)) Alkalinity A (mg/L CaCO3 ) Volatile acids, VA Influent solids VS reduction Biogas production (m3 /kg VSadded) Gas CH4 content H2 S content Gaseous hydrogen sulfide Gaseous ammonia Toxic organics Condition Close to 7.0 12–25 (avg. 17) 0.5–4 2000–3000 200–600 mg/L as acetic acid 2–7% TS 50–70% 0.3–0.5 Comments
J. Environ. Eng. Sci. Vol. 1, 2002
pH affects CH4 /CO2 and H2 S/S2 equilibrium and ammonia toxicity SRT in egg-shaped digesters as short as 10 d; SRT related to VS load through the percent TS in the influent Higher loads for carefully controlled and well-mixed process in egg-shaped digesters Add if VA/A increases over 0.25 Change in VA composition signals problems, particularly if propionic acid accumulates The higher, the better; thickening mandatory USEPA class B biosolids: 38% minimum Maximum theoretical CH4 production 0.35 m3 /kg CODremoved (volume at 0◦ C) Percent CH4 higher for high alkalinity and high pH H2 S toxic level >6% (Speece 1996); gaseous H2 S increases as pH decreases pH dependent; ≥1000 mg TS/L (total sulfides) well tolerated at high pH (McCartney and Oleszkiewicz 1993) At neutral pH, 2000–5000 mg Ntot /L tolerated; 300 mg/L un-ionized ammonia toxic (Poggi et al. 1991) Toxic levels to MPB (methane producing bacteria) very similar to levels for aerobic bacteria, except for chlorinated aliphatics, which inhibit MPB ten times more (Speece 1996) Add these metals if deficient; sulfide also needed, but excessive sulfide may precipitate metals (Speece 1996)
60–70% Trace «50 mg/L H2 S; 100% toxicity at H2 S > 250 mg/L <100 mg/L as NH3 un-ionized Lowest levels; chronic exposure 1 Fe, 0.1 Co, 0.1 Ni, 0.1 Se (dissolved!)
Metals need to be added as ¯ chlorides (mg/L)
thermophillic pre-treatment has been operating successfully in Sturgeon Bay, Wisconsin (T. Stebor, STS Consultants, personal communication, 2000). A number of WAS disintegration systems are presently being tested (e.g., Kopp et al. 1997). All show some improvement in the sludge digestion rate, allowing for shorter required SRT and larger VS reduction. The usually larger polymer demand for dewatering was compensated by the higher TS in the cake (Figs. 5D, 5F). Pilot-scale sodium hydroxide hydrolysis of WAS, followed by combined digestion with PS, showed that the SRT could be decreased from 25 d (without hydrolysis) to 10 d with alkaline pre-treatment (Mavinic et al. 1995). A new full-scale pulse power electric arc technology installed in Decatur, Alabama, was reported by a vendor to double methane production and volatile solids destruction (Greene 1995) (Fig. 5E). An added advantage was pathogen kill due to cell breakup, although the Ascaris eggs were found to be resistant. A number of bench-scale studies using ultrasound technology have demonstrated increased methane production and VS reduction, due to the availability of organics released after disintegration of cells walls (Tiehm et al. 1997). There are several proprietary processes (e.g., the Cambi process used in one of the London, England, plants and presently installed in Dublin, Ireland) that aim to disintegrate the activated sludge cells through pressurizing the sludge and then releasing the liquid into an expansion tank where the cells rupture. Data comparing the effectiveness of various pre-treatment options are not yet available; however, as these processes become more
are usually used) may actually lead to deterioration of sludge dewaterability (Vesilind 1979), the process has developed a poor reputation. The ambient-temperature digestion should have an adequate SRT of ≥25 d and provisions for denitrification, since prolonged oxidation leads to nitrification and pH decrease due to acidic product accumulation in the digesters. Operation in a sequencing batch reactor (SBR) mode, where denitrification takes place during the fill phase, may allow some alkalinity recovery; in extreme cases, alkalinity addition such as lime may be necessary. The latter may also help to retain phosphorus in the sludge, which otherwise would be returned to the front end of the plant in the supernatant (Anderson and Mavinic 1987). The use of thermophillic processes (56◦ C × 7.2 d = 400◦ C·d) is gaining popularity, based on successful performance of several installations in Western Canada. First popularized in Germany, autothermal (thermophillic) aerobic digestion (ATAD) relies on the generation of heat by the bacteria during oxidation when the feed sludge is concentrated to above 4% VS (45– 100 g/L COD). The reactors must be arranged in a series of at least two reactors (a number of three-reactor systems have been installed recently), with the temperature in the first being 50–55◦ C and in the second 55–70◦ C. However, there is some debate as to the usefulness of the higher temperatures, since excessive inactivation of all bacteria may lead to pathogen regrowth problems. Nitrification does not occur at these temperatures. Typical design conditions (WEF 1998) include the following: load 6–20 kg COD/(m3 ·d), air supply 4 m3 /(m3 ·h), and power for mixing 100–150 W/m3 . Canadian experience indicates complete pathogen inactivation, low specific oxygen uptake rate (SOUR) of 0.2–0.7 mg O2 /(g TS·h), and VS reduction exceeding 40% (Kelly et al. 1993; WEF 1998). A combination of the aerobic thermophillic digestion and anaerobic mesophilic digestion has been successfully implemented in Lebanon, Tennessee (Currie 1997) (Fig. 6). The process is an ideal upgrade of sludge to class A. The addition of a small (±1 d retention time) aerobic reactor is a relatively minor price to pay for pathogen inactivation without the use of chemicals. The process is run at reduced air requirements to maximize gas production in the anaerobic stage. Kelly (2000) compared ATAD to thermal drying and the thermochemical process for pathogen disinfection. ATAD was by far the most cost-effective technology and one that offered stabilization and pathogen inactivation. Digestion of sludge in a solid phase in an autothermal mode is called composting. A number of aerobic windrow composting plants operate in Canada with mixed success. Due to odor problems from open-air windrows the designers often opt for in-vessel composting. These have also been plagued with problems. Some, like the vertical cylindrical reactor systems, suffer because of excessive compaction at the bottom which leads to low oxygen permeability and an inferior product. In all composting operations, sludge has to be augmented with a bulking agent that increases the voids ratio such that total solids are at 40–50% TS. Wood chips are usually used as bulking agents; however, the organic fraction of solid wastes is
also becoming popular. A relatively simple process, composting is performed at temperatures of 47–60◦ C by a mixture of bacteria, actinomyces, and fungi. It requires a proper ratio of C:N, usually above 25:1. In mature compost the ratio should decrease to below 20:1 and SOUR should be below 150 mgO2 /(g TS·d). Composting can also be conducted in an anaerobic regime with considerable recovery of energy. A number of full-scale anaerobic composters operate in Europe, treating sludge with manure and the organic fraction of solid wastes. The solids level is kept usually above 35–40% TS, with typical gas yields of 200– 300 L/kg VS fed (Oleszkiewicz and Poggi 1997). Anaerobic composting or digestion may be followed by an aerobic curing or aerobic composting to oxidize the reduced products of the anaerobic processes. Alternative co-composting processes, such as the Canadian Subbor Process, utilize cell destruction to effect a higher degree of mineralization (Liu et al. 2000). Return sludge liquors In a modern BNR plant, sludge liquors present a significant potential problem, particularly if the initial ratio of COD to P in raw wastewater is below 40:1 and there are inadequate amounts of VFA in the influent. Sludge liquors from anaerobic and aerobic processes contain significant amounts of nitrogen (up to 1200 mg/L) and phosphorus (up to 200 mg/L), accounting for 10–30% of the influent load. In many plants special measures are taken to remove ammonia nitrogen by stripping or using in-stream nitrification with return activated sludge, so-called RAS reaeration (Reid Crowther & Partners Ltd. 1999). Head et al. (2000) demonstrated significant savings in activated sludge reactor size when nitrification of centrate was used, due to the nitrification seeding effect. Lime may be used to precipitate phosphorus. It should be stressed that treatment of return liquors must be justified by economic analysis, based on the balance of available carbon and the nitrification requirements. A unique method consists of struvite (magnesium ammonium phosphate or MAP) precipitation– crystallization. This normally nuisance, stone-like deposit that coats dewatering equipment can be made to precipitate within a sludge matrix through the use of magnesium oxide, temperature shock, or seeding in an upflow-type reactor. A full-scale upflow MAP-crystallization reactor was used to treat centrate from a 34 000 m3 /d plant practising BNR and anaerobic digestion (Fukase 1997).
Table 3. Predicted detention times for complete inactivation of Ascaris eggs in aerobic and anaerobic digestion processes (Reimers et al. 1999). Temp. (◦ C) 25 35 45 55 57 58 59 60 70 Aerobic digestion 130 d 90 d 50 d 10 d 2d <1 h <1 h <1 h <1 h Anaerobic digestion 74 d 53 d 30 d 9d 4d 3d 12 h <1 h <1 h
perhaps weaken them, but the infectivity remains. The USEPA (1994) requires the following: • Under class B, anaerobically digested sludge must meet the performance standards of a mean solids residence time and temperature between SRTs of 15 d at 35–45◦ C and 60 d at 20◦ C. • Under class A, anaerobic thermophillic digestion must meet the performance standards of an SRT of 10 d at 55◦ C or 1 d at 60◦ C. • Under class B, aerobically digested sludge must meet the performance standards of an SRT of 40 d at 20◦ C. • Under class A, liquid biosolids or sludges aerated under aerobic conditions must meet the performance standards of an SRT of 10 d at 55–60◦ C.
Oleszkiewicz and Mavinic Fig. 7. Modern dryers showing simultaneous disinfection and volume reduction (Oleszkiewicz and Reimers 1999b): (A) direct rotary drier; (B) direct fluidized-bed drier; (C) indirect disc drier.
85
ing since 1935 when the City of Milwaukee, Wisconsin, started distributing Milorganite. Despite the high price (the retail price was Can$2000/t TS in 1999 in Winnipeg), the operation appears to be economical. Production costs for dried biosolids usually do not exceed Can$200–450/t TS plus the cost of transportation and any additives. These costs are susceptible to the economy of scale. Even though the annual production is now approaching 0.5 Mt TS in North America, the market capacity is estimated at 50 Mt TS/year (Maestri and Graton 1996). The largest European indirect dryer was to have been commissioned in Barcelona, Spain, early in 2000. A similar indirect multipleshelves drying system (not shown in Fig. 7) for 7.5 t/h was also to have been commissioned in early 2000 for the San Miguel Plant in Sao Paulo, Brazil (Collier 1999). Anaerobic digestion precedes drying in the San Miguel facility, providing biogas. Due to transportation costs, drying appears to be the technology of choice for larger plants; however, recent developments in indirect drying have allowed plants with wastewater flows as small as 20 000 m3 /d to use this technology. Canadian facilities for drying unstabilized sludge operate in the Outaouais Region and at Smith Falls, Montréal, Quebec City, and Windsor. The dryers installed at the Ashbridges Plant in Toronto will treat half of their anaerobically digested and dewatered biosolids (Iamonaco 2000). The field of disinfection and volume reduction is currently expanding rapidly. One of the most promising technologies is the Unity process (J.C. Burnham, J.C. Burnham Company, Naples, Florida, personal communication, 2000) in which sludge is used to quench the commercial fertilizer producing reaction of anhydrous ammonia with sulfuric acid. The product sells at US$50/t
TS on the agricultural market, and the company makes money when they get US$100/t tipping fee. The City of New York sends trainloads of sludge for processing into this organic ammonia fertilizer. Slowly the thinking in the industry is changing towards the value-added product, with a more sincere look at the needs of the user.
J. Environ. Eng. Sci. Vol. 1, 2002 Table 4. Polymer dose (kg/t TS) optimized for various machines in three different cities in Denmark (Andreasen and Nielsen 1993). Ejby Molle (WAS, 1% TS) Belt filter press Centrifuge Press centrifuge A Press centrifuge B Membrane press 2 10 6.2 3 4.6 Schol (mixed, 4% TS) 3 5.2 4.5 8.1 5.6 Marselisborg (digested sludge, 4% TS) 5.5 10 13 11 4.3
ber of machines dewatering digested biosolids from the BNR plant in Pruszkow, Poland, have clearly demonstrated the superiority of a press centrifuge over other dewatering equipment, mainly because of high transportation costs and high tipping fees (M. Bieniowski, personal communication, 2000). The choice of dewatering device is also defined by the size of the plant. Smaller plants require mechanical simplicity, minimum supervision, and easy maintenance, whereas larger plants require the driest cake achievable and the lowest cost per ton. A big factor is the contact of the operator with the sludge and sludge odors. Since attention to the operator’s comfort traditionally favored closed centrifugation systems, other manufacturers are beginning to hermetically seal the belt filter presses (BFP) and the whole sludge conditioning and delivery system. Two new (on line in year 1999) advanced BNR plants for the cities of Gdansk and Krakow in Poland are operating hermetically sealed belt filtration machines for both thickening and dewatering.
Biosolids process train
Any full process train will be determined by the required quality for final disposal. The assumption that class A solids are to be produced considerably narrows the choice. When maximum stabilization with disinfection is required, thermophillic processes will be the prime choice, aerobic for smaller plants and anaerobic for larger plants. Composting is an option; however, it is far less developed or successful than ATAD or thermophillic anaerobic digestion. The introduction of sludge drying into smaller scale plants will simplify the train by eliminating the need for biostabilization; however, care is needed to keep the solids from being wetted prematurely due to odor generation potential. In all cases marketing becomes the ultimate issue, determining the economic success of land application or value-added processing. Biosolids disinfection is slowly making inroads; however, relatively high operating costs may offset the low start-up costs. Unless a full biosolids to soil process is developed (such as some alkaline processes), chemical treatment would be, at best, a temporary measure. The costs of biosolids treatment are substantial but do decrease with size. Economy of scale drives municipalities to consolidate solids processing from smaller plants into one group facility (e.g., in Winnipeg, New York, or San Diego). The cen-
Oleszkiewicz and Mavinic Fig. 8. Optional process trains for separately thickened biosolids from a large treatment plant requiring class A product for land application.
87
with larger plants benefiting from an array of new devices offering sludge cakes as dry as 22–45% TS; this will depend on the quality of sludge, polymer conditioning program, and machine parameters. Despite all of the recent advances, the axiom first put forth by Vesilind (1979), “there are no typical sludges,” still holds true and can be expanded to say that there are no universal “proper” residuals treatment and management schemes. This should spell relief to all design consultants fearing that the “off-the-shelf” solution is around the corner. Emphasis on cost reduction with simultaneous demand for an excellent quality product are still a formidable task for any municipality and design engineer.
Summary and conclusions
The current emphasis on sustainable development has led to the renaming of “sludge” after stabilization and dewatering to “biosolids,” with far-reaching technological consequences. Treated as a valuable resource, the sludge today is processed through a variety of novel unit operations leading to a safe, aesthetically pleasing, and sought-after product. The product mentality changes the approach and the market-driven technologies offering designer organic fertilizer are emerging. Treatment plant personnel must now be concerned with the ultimate quality control, not only in-plant but also of the solids that are being received and sent out. The design engineer is concerned first with the ultimate disposal and utilization of the biosolids, providing at least two options for the final disposal. Volume reduction, stabilization or vector attraction reduction, and pathogen inactivation are the key goals; process trains combining them into one unit process are the target technologies. Drying may now be applied at much smaller plants because of the introduction of indirect dryers which do not have the air pollution problems of the direct dryers still used at larger plants. Stabilization in newer plants is preferably combined with disinfection at thermophillic temperatures, in anaerobic and particularly in aerobic regimes. Dewatering is now available with drying bags and vacuum drying beds for the smallest plants,
88 Head, M., Oleszkiewicz, J., and Kos, P. 2000. Nitrification of highammonia reject water for improved efficiency of the main stream treatment train. In Proceedings of Water Environment Federation Technical Conference WEFTEC 2000, Anaheim, Calif., 14–18 Oct. 2000. Hilton, B.L., and Oleszkiewicz, J.A. 1988. Sulfide toxicity to the anaerobic treatment process. Journal of the Environmental Engineering Division, ASCE, 114(6): 1377–1391. Iamonaco, C. 2000. Biosolids processing techniques. Available via City of Toronto Website: Sewers and Drains. Jardin, N., and Popel, H.J. 1996. Influence of the enhanced biological phosphorus removal on the waste activated sludge production. Water Science and Technology, 34(1–2): 17–23. Kelly, H.G. 2000. Comparing North American biosolids treatment of thermophilic digestion, thermal–chemical and heat drying technologies. Dayton & Knight, Vancouver, B.C. Kelly, H.G., Melcer, H., and Mavinic, D. 1993. Autothermal thermophilic digestion of municipal sludge: a one year full scale demonstration project. Water Environment Research, 65(7): 849–861. Kiely, P. 1997. Environmental engineering. McGraw-Hill, New York. Koers, D.A., and Mavinic, D.S. 1977. Aerobic digestion of waste activated sludge at low temperatures. Journal Water Pollution Control Federation, 49(3):460–468. Kopp, J., Muller, J., Dichtl, N., and Schwedes, J. 1997. Anaerobic digestion and dewatering characteristics of mechanically disintegrated excess sludge. Water Science Technology, 36(11): 129–136. Lishman, L.A., Legge, R.L., and Farquar, G.J. 2000. Temperature effects on wastewater treatment under aerobic and anoxic conditions. Water Research, 34(8): 2263–2276 Liu, H., Vogt, G.M., and Holbein, B.E. 2000. Subbor anaerobic digestion process for solid organic wastes: elimination of a major source of water-borne contaminants. In Abstracts of the 16th Regional Conference of the Canadian Association of Water Quality, Ottawa, 17 Nov. 2000. Maestri, T., and Graton, P. 1996. The case for pelletization. In Proceedings of the 10th Annual Residuals and Biosolids Management Conference, Water Environment Federation, Denver, Colo., pp. 20.25– 20.31. Mavinic, D.S., Knezevic, Z., Anderson, B.C., and Oleszkiewicz, J.A. 1995. Fate of nutrients during anaerobic co-digestion of sludges from a biological phosphorus removal process. Environmental Technology, 16: 1165–1173. McCartney, D.M., and Oleszkiewicz, J.A. 1993. Competition between methanogens and sulfate reducers: effect of COD:sulfate ratio and acclimation. Water Environment Research, 65: 655–666. Metcalf and Eddy, Inc. 1991. Wastewater engineering. 3rd ed. McGraw-Hill Inc, Boston. Oleszkiewicz, J.A. 1998. Sludge management: decision-makers handbook. U.S. AID/LEM, Krakow, Poland. Oleszkiewicz, J.A. 1999a. Production, quantity and characterization of sludge. In Biosolids: Regulatory, Process and Utilization Technologies, Proceedings of the Western Canada Water Environment Association Seminar, Winnipeg. Oleszkiewicz, J.A. 1999b. Process alternatives. In Biosolids: Regulatory, Process and Utilization Technologies, Proceedings of the Western Canada Water Environment Association Seminar, Winnipeg.
J. Environ. Eng. Sci. Vol. 1, 2002 Oleszkiewicz, J., and Poggi, H. 1997. High solids anaerobic digestion of mixed municipal and industrial solids. Journal of Environmental Engineering, ASCE, 123: 1087–1092. Oleszkiewicz, J.A., and Reimers, R.S. 1999a. Biological stabilization: anaerobic digestion. In Biosolids: Regulatory, Process and Utilization Technologies, Proceedings of the Western Canada Water Environment Association Seminar, Winnipeg. Oleszkiewicz, J.A., and Reimers, R.S. 1999b. Biosolids drying. In Biosolids: Regulatory, Process and Utilization Technologies, Proceedings of the Western Canada Water Environment Association Seminar, Winnipeg. Orlando, L. 1997. The sludge scam: should sewage sludge fertilizer your vegetables. ReSource, Jamaica Plain, Mass. Outwater, A.B. 1994. Reuse of sludge and minor wastewater residuals. Lewis Publishers, Boca Raton, Fla. Poggi, H., Tingley, J., and Oleszkiewicz, J. 1991. Inhibition of growth and acetate uptake by ammonia in batch anaerobic digestion. Journal of Chemical Technology and Biotechnology, 52: 135–143. Poling, M., and Reynolds, D. 1996. Space-saving solids handling. Water Environment and Technology, 9(1): 25–39. Reid Crowther & Partners Ltd. 1999. The City of Winnipeg nitrification study. Reid Crowther & Partners Ltd., Winnipeg. Reimers, R., DeSocio, E.R., Bankston, W.S., and Oleszkiewicz, J.A. 1999. Current/future advances in biosolids disinfection processing. In Biosolids: Regulatory, Process and Utilization Technologies, Proceedings of the Western Canada Water Environment Association Seminar, Winnipeg. Smith, C. 1999. Blue Riband Works. Water & Environment Manager, CIWEM, 4(5): 10. Speece, R. 1996. Anaerobic biotechnology for industrial wastes. Archae Press, Nashville, Tenn. Tata, P., Lue-Hing, C., Bertolucci, J., Sedita, S., and Knaft, G. 2000. Class A biosolids production by a low cost conventional technology. Water Environment Research, 72: 413–422. Tiehm, A., Nickel, K., and Neis, U. 1997. The use of ultrasound to accelerate the anaerobic digestion of sewage sludge. Water Science Technology, 36(11): 121–128. USEPA. 1994. A plain English guide to the EPA part 503 biosolids rule. EPA/832/R-93/003, U.S. Environmental Protection Agency, Washington, D.C. Vesilind, A. 1979. Treatment and disposal of wastewater sludges. Ann Arbor Science, Ann Arbor, Mich. WEF. 1996. Developing source control programs for commercial and industrial wastewaters. OM-4, Water Environment Federation, Alexandria, Va. WEF. 1998. Design of municipal wastewater treatment plants. MOP-8, Water Environment Federation, WEF–ASCE, Alexandria, Va. WEF Biosolids. 1998. Colorado County lifts ban on biosolids land application. Water Environment Federation WEF Biosolids Technical Bulletin, 4(4). WEF Biosolids. 1999. Torpey recirculation process increases digester capacity in Phoenix. Water Environment Federation WEF Biosolids Technical Bulletin, 5(3).