a n d d esig n ed for th e sepa ra tion of th e solid from th e liq u id fra ction of (fa eca l) slu d g e from la trin es,
sep tic ta n k s, biog a s rea ctors, trick lin g filters, etc. Slu d g e is d ried n a tu ra lly by a com bin a tion of
p ercola tion a n d ev a p ora tion . W h en p la n ts a re p resen t, ev a p ora tion is en h a n ced by tra n sp ira tion
(ev a p otra n sp ira tion ). Th e p ercola te still con ta in s p a th og en s a n d n eed s to be collected for trea tm en t or
con trolled reu se. After trea tm en t in p la n ted d ry in g bed s (h u m ifica tion bed s), th e d ried slu d g e, a
n u trien t-rich soil am en d m en t, ca n be d irectly u sed in a g ricu ltu re. If u n p la n ted slu d ge d ry in g bed s are
u sed , a d d ition a l trea tm en t by com p ostin g m a y be foreseen . Un p la n ted d ry in g bed s n eed to be
d eslu d g ed before fresh slu d g e is a p p lied , w h ile h u m ifica tion bed s ca n be filled u p in term itten tly
req u irin g d eslu d g in g on ly on ce ev er 5 to 10 y ea rs. Dry in g bed s a re rela tiv ely ea sy to con stru ct a n d
sim p le to m a in ta in , a lth ou g h la rg e su rfa ce a rea s a n d m a n - or m ech a n ica l p ow er is req u ired for
reg u la r d eslu d g in g .
In Out
Faecal Sl udge
Compost (for furt her off-si t e compost i ng), Soi l , Tr eat ed Wat er, Fer t i gat i on Wat er or Wat er r equi er i ng fur t her t r eat ment
Al l or gani c degradat i on pr ocesses produce sl udge (SASSE & BORDA 1998). Sl udges fr om wast ewat er col l ect i on or t reat ment uni t s are of var i abl e consi st ency (f rom sept i c t anks, smal l and l ar ge scal e anaer obi c di gest er s,
anaerobi c di gest er s for organi c wast e, anaerobi c baffl ed r eact ors, wast ewat er st abi l i sat i on pond syst ems, et c.). General l y, t hey do not have a hi gher sol i ds cont ent t han 2 t o 10 % and can t her efore not be t ransport ed easi l y wi t h si mpl e equi pment . Apar t fr om t hi s, sl udge i s cont ami nat ed and occupi es l ar ge vol umes f or st or age (SASSE & BORDA 1998). Ther ef or e i t i s bet t er t o dr y or dewat er sl udge before fur t her use or dumpi ng. Sl udge dr yi ng beds ar e one of t he si mpl est and ol dest t echni ques for sl udge dewat er i ng (SANIMAS 2005). They ar e a si mpl e means t o reduce t he vol ume of t he sl udge and pr epar e r euse as fer t i l i ser.
Sl udge dryi ng beds ar e basi cal l y i mpermeabl e beds fi l l ed wi t h di ff er ent l ayers of gr avel and sand. Dr ai nage pi pes ar e i ncor por at ed i n t he bot t om of t he beds.
Let t uce f ar m f er t i l i sed wi t h saf e compost made out of f aecal sl udge t r eat ed i n dr yi ng beds at Gyenyasi f ar mer s associ at i on i n Kumasi . Sour ce: ERIKSEN-HAMEL & DANSO (2008)
Sl udge i s appl i ed i n l ayers on t op of t he gr avel beds and i s nat ur al l y dr i ed. The bed can be ei t her pl ant ed or unpl ant ed. Aft er t reat ment i n pl ant ed dr yi ng beds (humi fi cat i on beds), t he dr i ed sl udge, a nut r i ent -r i ch soi l amendment (or bi osol i d), can be di rect l y used i n agr i cul t ur e. If unpl ant ed sl udge dr yi ng beds ar e used, addi t i onal t r eat ment by compost i ng may be for eseen. Thi s al l ows t o st abi l i se t he sl udge and t o ret ur n nut ri ent s and or gani c mat t er i nt o t he soi l for agr i cul t ur e. The per col at e cont ai ns pat hogens and i s col l ect ed i n t he drai nage pi pes. It can be r eused, but t hen r equi res t r eat ment .
Unpl ant ed and pl ant ed dr yi ng bed. Sour ce: TILLEY et al . (2008)
A t r adi t i onal sl udge dr yi ng bed can be used for dewat eri ng and dr yi ng of faecal sl udge and anaerobi c di gest er r esi due. Anaer obi c sani t at i on syst ems (e. g. l at r i nes, sept i c t anks, aqua pr i vi es, anaerobi c baff l ed react or s, bi ogas react or s) pr oduce l ess sl udge t han aer obi c t r eat ment s (e.g. f rom t r i ckl i ng fi l t er s, act i vat ed sl udge) and anaerobi c sl udge al so dr i es bet t er and r esul t s i n fewer odours as i t i s more st abi l i sed. Sl udge i s dewat er ed by per col at i on, whi l e t he sol i d f ract i on r emai ns on t he fi l t er sur face and i s dr i ed by nat ur al evaporat i on. In pl ant ed dryi ng beds, t he pl ant s enhance evaporat i on by t ranspi r at i on. Thi s process i s cal l ed evapot r anspi rat i on.
In pr i nci pl e t he const r uct i on of unpl ant ed and pl ant ed sl udge dr yi ng beds i s si mi l ar t o gr avel / sand fi l t er s, st ar t i ng wi t h an i mper meabl e shal l ow pond, dr ai nage pi pes i n t he bot t om (perf or at ed PVC pi pes or hol l ow bl ocks), and di ffer ent drai nage l ayer s. The bed f rame i s usual l y made f rom concr et e or a pl ast i c l i ner wi t h t he bot t om sur faces sl i ght l y sl oped i n or der t o faci l i t at e per col at i on and dr ai nage. The l owest l ayer of t he bed consi st s of a dr ai nage st r at um made of coar se gravel . Upper l ayers consi st of di ffer ent sand and gr avel wi t h fi ner grai n si ze at t he t op. For t he sl udge appl i cat i on, a spl ash spat e may be used t o avoi d sput t er i ng.
Indi cat i on about t he di amet er of t he medi um and t he hei ght of t he di f ferent gr avel and sand l ayer s var y dependi ng on t he sour ce (SASSE & BORDA 1998; STRAUSS & MONTANGERO 2004). General l y, t he coar se gr avel l ayer (grai n di amet er : 15 t o 50 mm) i s wi t hi n 20 t o 30 cm of hei ght . A gr avel l ayer (di amet er of 7 t o 15 mm) of 10 t o 15 cm fol l ows t hi s l ayer. Thi s l ayer can be fol l owed of a si mi l ar one of sl i ght l y l ower di amet er (see pi ct ure bel ow, STRAUSS & MONTANGERO 2002), but t hi s i s not compul sor y (SASSE 1998). There shoul d be a fi nal sand l ayer of 25 t o 30 cm. As some sand wi l l be l ost each t i me t he sl udge i s manual l y removed, i t may be hi gher (STRAUSS & MONTANGERO 2004; TILLEY et al . 2008).
Unpl ant ed dr yi ng beds i n Ghana (l ef t ) and r ai n pr ot ect ed unpl ant ed dr yi ng bed at t he Ar cat a wast ewat er t r eat ment pl ant , USA (r i ght ). Sour ce: EAWAG/ SANDEC (2008); HUMBOLDT EDU (2008)
The sl udge i s appl i ed i n a bat ch mode about once per week i nt er val s i n l ayers of no more t han 20 t o 30 cm. About 50 t o 80 % of t he i ni t i al vol ume i s removed by percol at i on (STRAUSS & MONTANGERO 2002), r esul t i ng i n t ot al sol i d (TS) cont ent of 20 t o 70 % (STRAUSS & MONTANGERO 2004) dependi ng on t he l ocal weat her condi t i ons and cl i mat e. In r egi ons wi t h f requent rai nfal l , cont our bounds can pr event surf ace runof f t o ent er t he ponds and cover i ng t he dryi ng beds wi t h a r oof may be consi der ed.
Dr yi ng i nduces par t i al pat hogen r emoval . However, t he dr i ed sl udge st i l l may cont ai n pat hogens, par t i cul ar l y Hel mi nt h eggs, and shoul d t her efore be handl ed car ef ul l y and r ecei ve fur t her t r eat ment such as compost i ng or prol onged st or age befor e use i n agr i cul t ur e. The percol at e fr om dewat er i ng cont ai ns al so pat hogens, mai nl y bact er i a and vi ruses and has t o be furt her t r eat ed as wel l (STRAUSS et al . 1997). In t he case of fr equent appl i cat i on of sl udge and t o enhance r et ent i on t i mes, t wo or mor e dr yi ng beds can be const r uct ed i n par al l el and used al t ernat el y.
On an annual basi s, about 100 t o 200 kg TS/ m2 can be appl i ed on a dr yi ng bed. Dryi ng t akes 10 t o 20 days. Land
r equi rement s ar e 0.05 m2 per capi t a for a 10 days cycl e (STRAUSS & MONTANGERO 2002). Bef or e fr esh sl udge i s appl i ed, dr i ed sl udge needs t o be desl udged and br ought t o a compost i ng si t e, what makes i t a rat her l abor i ous t reat ment opt i on.
Cr osscut of a pl ant ed dr yi ng bed. Sour ce: STRAUSS & MONTANGERO (2004)
Pi l ot beds of t he VFCW devel oped f or f aecal sl udge dewat er i ng st udy i n sub-Sahar an count r i es. Sour ce: KONE & KENGE (2008)
Pl ant ed sl udge dr yi ng beds, al so desi gnat ed as r eed beds or const r uct ed wet l ands, coul d mi ni mi se t he need f or f r equent r emoval of dr i ed sl udge as t hey can be oper at ed f or sever al year s bef or e sl udge r emoval becomes necessar y. Sour ce: EAWAG/ SANDEC (2008)
As for unpl ant ed beds, pl ant ed beds consi st of an i mper meabl e shal l ow pi t f i l l ed wi t h di ffer ent l ayer s of coar se t o f i ne sand. General l y, t her e ar e t hr ee l ayers, st ar t i ng wi t h a l ar ge gr avel l ayer (di amet er of 20mm) of 25 cm hei ght , fol l owed by a fi ne gr avel l ayer wi t h gr anul es of 5 mm i n di amet er (al so 25 cm hei ght ) and fi nal l y cover ed wi t h a sand l ayer of some 10 cm (EAWAG/ SANDEC 2008). Unl i ke unpl ant ed beds, pl ant ed beds do not need desl udgi ng befor e each new appl i cat i on as t he r oot syst em of t he pl ant s mai nt ai ns t he permeabi l i t y. The sl udge i s added i nt er mi t t ent l y once a week and onl y r emoved ever y 5 t o 10 year s (STRAUSS et al . 1997; STRAUSS & MONTANGERO 2002). Once t he sl udge i s r emoved i t i s wel l mi ner al i sed and has a soi l -l i ke st ruct ure wi t h a TS cont ent of 40 t o 70 % (STRAUSS & MONTANGERO 2002). Ther efore, pl ant ed sl udge dr yi ng beds ar e al so cal l ed humi fi cat i on beds. It i s best t o st op appl yi ng sl udge one or t wo year s befor e removi ng i t (whi l e a par al l el bed r ecei ves t he sl udge). In such a way, t he humi fi ed sl udge i s near l y pat hogen fr ee and can be reused di r ect l y as bi osol i d i n agr i cul t ur e.
Per col at e qual i t y al so consi der abl y i mproves by t he pr esence of pl ant s but may st i l l r equi r e a pol i shi ng t reat ment (STRAUSS et al . 1997).
Pl ant ed beds can r ecei ve about 250 kg TS/ m2 per year (STRAUSS & MONTANGERO 2002).
H e a lth As p e cts
Fresh sl udge i s gener al l y hi ghl y pat hogeni c and shoul d be handl ed wi t h car e. Al so a cer t ai n di st ance t o t he i nst al l ed beds (especi al l y when sl udge has been recent l y appl i ed) shoul d be r espect ed. Per col at es f rom sl udge dryi ng beds cont ai n al so pat hogens and need t o be fur t her t r eat ed.
Sl udge fr om unpl ant ed beds shoul d be compost ed before r euse t o enhance pat hogen r emoval .
hi gh. In any case, t he ponds shoul d be seal ed t o pr event i nfi l t rat i on of t he pat hogen cont ai ni ng per col at e and a
Exhaust i v e report on t echnologi cal, operat i onal and economi c aspect s of decent rali sed w ast e w at er t reat ment syst ems. Spreadsheet examples support t he reader i n desi gni ng and planni ng w ast e w at er t reat ment syst ems component s.
STRAUSS, M. ; LARMIE, S. A. ; HEINSS, U. (1997): Treat ment of sludges f rom on-si t e sani t at i on — Low -cost opt i ons. In: Wat er Sci ence and Technology 6, 129-136. URL [ Accessed: 23. 06. 2010] . PDF
HUMBOLDT EDU (Edi t or) (2008): Arcat a's Wast ew at er Treat ment Plant & The Arcat a Marsh and Wi ldli f e Sanct uary. Arcat a: Env i ronment al Resources Engi neeri ng, Humboldt St at e Uni v ersi t y. URL [ Accessed: 22. 05. 2012] .
