Table 8. Vacuum filter performance for processing municipal sludges conditioned by ferric chloride and lime. a

Transcription

1 Tble 8. Vcuum filter performnce for processing municipl sludges conditioned by ferric chloride nd lime. Type of Sludge Solids Loding (lb/hr-ft 2 ) Percent Solids Cke Fresh primry Fresh primry nd wste ctivted sludge Fresh primry nd wste ctivted sludge (pure oxygen) Fresh primry nd trickling filter humus Digested primry (nerobic) Digested primry nd wste ctivted sludge (nerobic) Adpted from The EPA, Sludge Tretment nd Disposl, FPA Process Design Mnul Wshington, D.C., Centrifugtion Sludge centrifugtion for dewtering purposes is the sme s for the sludge thickening process. The sludge is fed into rotting solid bowl centrifuge where it is centrifuged nd collected on the inner wll of the bowl. The percentge of dry solids in the sludge cke removed rnge from 10-30%, depending on the previous processes used before dewtering (Reynolds nd Richrds 1996). Once the tretment is performed to produce the end products of effluent wter nd biosolids, the specific question of "Wht to do with it now?" rises. The wter effluent is usully treted with chlorine or UV light s finl disinfection step before being returned to the receivingwters or pplied to griculturl lnd. The nture of the conditioned biosolids poses greter disposls concerns thn tht of the wter. 4.0 CHARACTERIZATION As ws stted erlier, vriety of combintions of processes cn be used to tret wstewter nd the choice of systems is dependent on the individul need of the plnt. One difficulty resulting from different unit processes is the bility to ccurtely chrcterize the sludge t different points in the system. This section will present nutrient nd metl quntities s well s physicl chrcteristics of biosolids produced from vrious processes. 31

2 Biosolids re comprised of three mjor components; ded orgnic cell mss from microbes nd bcteri, decomposed orgnic solids nd inert mteril. Tble 9 shows nutrient levels found in wstewter biosolids subjected to different tretment processes (Stein, et l., 1995). The ctul mount of biosolids produced in tretment fcility is dependent on mny opertionl prmeters, few of which include temperture, mixing, recircultion, velocity, flow nd detention time. These prmeters re set by the opertor to promote efficient opertions of the processes. Tble 10 presents chrcteristics of biosolids from different unit processes tht re commonly used. 4.1 Prt 503 As result of the United Sttes Government mending the Clen Wter Act in 1987, the U.S. Environmentl Protection Agency (EPA) developed new regultion to protect public helth nd the environment from ny resonbly nticipted dverse effects of certin pollutnts tht might be present in biosolids. This regultion, The Stndrds for the Use or Disposl of Sewge Sludge (Title 40 of the Code of Federl Regultions [CFR], Prt 503) ws published in the Federl Register in This regultion will be referred to in this report s Prt 503. Prt 503 estblishes requirements for the finl use or disposl of biosolids when they re pplied to lnd s soil conditioner or fertilizer, plced on surfce disposl site, fired in n incinertor, or plced in MSW lndfill (Wlker, 1994). The Georgi Environmentl Protection Division drws upon the EPA s Prt 503 regultions s the bsis for Georgi s regultions. The EPA provides no funding for the sttes to tke over or dminister Prt 503, thus mking it selfimplementing responsibility for ech stte. 32

3 Tble 9. Nutrient levels in sewge sludge from different tretment processes. Sludge Tretment Nutrient b Process c No. of Smples Rnge Medin Men Orgnic C (%) Anerobic Aerobic Other All Totl N (%) Anerobic Aerobic Other 68 < All 191 < NH + 4-N (mg/kg) Anerobic , ,400 Aerobic , Other , ,200 All , ,540 NO - 3-N Anerobic , Aerobic Other All , Totl P (%) Anerobic Aerobic Other 65 < All 189 < K (%) Anerobic Aerobic Other All N (%) Anerobic Aerobic Other All C (%) Anerobic Aerobic Other All Adpted from Stein (1994). b Concentrtion nd percent composition re on dried solids bsis c "Other" includes lgooned, primry, tertiry, nd unspecified sludges. "All" signifies dt for ll types of sludges 33

4 Tble 10. Typicl quntities of sludge produced by different unit processes. Unit Processes Typicl Quntity of Sludge Specific Specific gl/mg tons/mg of Wstewtewter of Wste- ft 3 Moisture Grvity Grvity /1000 (%) of Sludge of persons Solids Sludge lb/mg of Wstewter lb/1000 persons dily Primry Sedimenttion: Undigested 2, , Digested in seprte tnks 1, Digested & dewtered, snd beds d Digested & dewtered, vcuum filters Trickling Filter c Chemicl Precipittion: 5, , Dewtered on vcuum filters e , Primry Sedimenttion & Act. Sludge: Undigested 6, , Undigested & dewtered, vcuum filter 1, , Digested in seprted tnks 2, , Digested & dewtered, snd beds f , Digested & dewtered, vcuum filters , Activted Sludge b : Wet Sludge 19, , Dewtered on vcuum filters , Dried by het dryers , Septic Tnks, digested: Adpted from ASCE & WPCF (1977), Wstewter flow of 100 gpcd ( /person d) nd 300 mg/l, or.25 lb/dy/cp of suspended solids in wstewter. Sludge produced vries ccording to type nd extent of industril connections, collection system, & plnt loction. b Rw wstewter dischrge directly to ertion tnks c With secondry sedimenttion d Bsed on density of 33 lb/cu ft (530 kg/m3) e Bsed on density of 62 lb/cu ft (1000 kg/m3) f Bsed on density of 19 lb/cu ft (305 kg/m3) Dry Solids One mjor concern when focusing on the potentil reuse cpbility is the metls content of the biosolids. Prt 503 sets ceiling for metls concentrtion in biosolids tht re going to be used on the lnd in one form or nother s shown in Tble 11. A description of the potentil reuse methods used in Georgi will be discussed in the next section; Current Biosolids Mngement Prctices. 34

5 Tble 11. Pollutnt limits. Pollutnt Ceiling Concentrtion Limits for ll Biosolids Applied to Lnd (mg/kg) b Pollutnt Concentrtion Limits for EQ d nd PC e Biosolids (mg/kg) Arsenic Cdmium Chromium 3,000 1,200 Copper 4,300 1,500 Led Mercury Molybdenum 75 - c Nickel Selenium Zinc 7,500 2,800 Applies to: All biosolids tht re lnd pplied Bulk biosolids nd bgged biosolids f From Prt 503 Tble 1, Section Tble 3, Section Adpted from Wlker (1994) b Dry weight bsis c As result of the Feb 25, 1994, Amendment to the rule, the limits for molybdenum were deleted from the Prt 503 rule pending EPA reconsidertion d EQ: Exceptionl Qulity e PC: Pollutnt Concentrtion f Bgged biosolids re sold or given wy in bg or other continer All wstewter fcilities monitor their biosolids to ensure complince with this regultion. The EPA s 1990 Ntionl Sewge Sludge Survey (NSSS) nlyzed smples of 412 pollutnts or nlyses from 177 POTW s using t lest secondry tretment processes, including the ten metls regulted by Prt 503 (Stein, Boulding et l., 1995). Tble 12 shows the verge sludge concentrtions of ech of Prt 503 pollutnts. Biosolids re ctegorized by qulity s they relte to disposl methods. The solids re clssified s being either Excellent Qulity (EQ), clss A or clss B. Clss A nd B refers to the Prt 503 pthogen stndrds tht must be met t the time of ppliction or disposl. The EQ biosolids hve nerly undetectble pthogen levels, controlled vector conditions, low metl content nd stble orgnic mteril so there is little odor. EQ mteril cn be sold or pplied to lnd without ny site restrictions. Clss A biosolids hve very low pthogens nd typiclly low metls concentrtions, however, s it stnds Prt 503 only requires tht metls be below the ceiling concentrtion limits. 35

6 Tble 12. Men concentrtions of metls in sewge sludge compred to Prt 503 ceiling concentrtion limits. Men Concentrtion (mg/kg, DW) Prt 503 Pollutnt Ceiling Concentrtion Limits (mg/kg,dw) Metl Arsenic Cdmium Chromium Copper Led Mercury Molybdenum Nickel Selenium Zinc Adpted from Stein (1994) EPA's 1990 Ntionl Sewge Sludge Survey (NSSS) nlyzed smples of 412 pollutnts or nlytes from 177 POTW's using t lest secondry tretment processes, including the 10 metls regulted by Prt 503 for lnd ppliction. Clss B biosolids hve higher pthogen contents tht Clss A, but pthogens re reduced to levels not likely to pose thret to public helth nd the environment under specific use conditions (Stein, et l., 1995). There could be possible problems with vector disturbnces if proper mngement prctices re not implemented. There re griculturl lnd ppliction restrictions in plce for clss B to minimize the interction tht humns nd nimls cn hve with the lnd pplied biosolids until possible pthogen levels re reduced to below detectble levels. Clss B solids, requiring site permit, hve considerble restrictions on the type of humn food crops cn be plnted nd on the plnting times fter ppliction. Agin, metls content of Clss B biosolids re typiclly low but only requirement is to be below ceiling threshold. Biosolids tht do not rech Clss B stndrds cnnot be pplied to lnd without further tretment since it does not meet the pthogen, metl, or vector reduction stndrds. Tble 13 summrizes the clssifiction requirements for lnd ppliction biosolids clsses. 36

7 Tble 13. Regultory clssifictions for lnd pplied biosolids. Exceptionl Qulity Clss A Clss B Metls Low Low or Medium Low or Medium Pthogens b Low Low Medium Vector Attrction Low Low Low Odor Low Low Low Permits None Yes Yes Fertilizer Yes Yes Yes Soil Amendment Yes Yes Yes Mrketble Yes Yes No Low- Below EPA Tble 3 Pollutnt Concentrtions Medium- Between EPA Tble 1 Ceiling Concentrtion Limits & Tble 3 Pollutnt Concentrtions b Low is <1,000 MPN/g TS Medium is <2,000,000 MPN/g TS (MPN/g TS = Most Probbly Number per grm of Totl Solids) 37