The use of biosolids from wastewater treatment
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a gr icu ltu r e took pla ce, m a in ly du e to th e fol-low in g t wo r ea son s: a r t ifi cia l fer t ilizer s w er e ga in in g gr ou n d fa st ; a n d a st r on gly -in cr ea s-in g en v ir on m en t a l aw a r en ess w it h s-in t h e Ger m a n p op u la t ion h a d led t o m or e cr it ica l exa m in a t ion of slu d ge con st it u en t s. In t h is con t ext , I wou ld lik e t o r em in d you for in st a n ce of t h e h ea t ed a r gu m en t s over t h e con t en t of d ioxin es in slu d ge d u r in g t h e la t e 1980s.
Du r in g t h e 1990s h ow ever, t h e u se of slu d ge in a gr icu lt u r e gr ew in im p or t a n ce a ga in . Over 800,000 t on s of d r y solid m a t t er w er e em p loyed in a gr icu lt u r e in t h e r eu n it ed Ger -m a n y in 1992, r ou gh ly 25 p er cen t of t h e t ot a l q u a n t it y. T h e a r ea s a ffect ed cover a r ou n d 500,000 h ect a r es of a r a ble la n d .
Why should sludge be utilized in
agriculture?
T h e id ea of feed in g va lu a ble or ga n ic m a t t er, p r im a r ily n it r ogen a n d p h osp h or u s, b a ck in t o a n a t u r a l cycle for m s t h e b a sis of u sin g slu d ge in a gr icu lt u r e. On e of t h e p osit ive sid e-effect s is t h e p r ot ect ion of n a t u r a l r esou r ces, esp ecia lly p h osp h or u s – w h ich w ill b e d is-cu ssed in d et a il.
Ad d it ion a lly, cost ca lcu la t ion s h ave sh ow n t h a t u sin g slu d ge in a gr icu lt u r e p r oves t o b e a m or e econ om ica l a lt er n a t ive t h a n ot h er k in d s of slu d ge d isp osa l.
Fertilizing value of sludge
T h e n it r ogen , p h osp h or u s a n d , in som e ca ses, lim e con t en t of slu d ge, d e p en d s p r im a r ily on t h e k in d of sew a ge – a n d slu d ge t r ea t m en t . T h e d ew a t er in g of slu d ge ca u ses a sign ifi ca n t d ecr ea se in t h e n it r ogen con t en t , a lon g w it h t h e r em ova l of excess w a t er. Con ver sely, lim e is q u it e fr eq u en t ly a d d ed in t h e p r ocess of d ew a t er in g – a ga in , t h is r esu lt s in t h e a ccu m u la t ion of va lu a ble n u t r ien t s.
T h e elim in a t ion of p h osp h or u s in t h e cou r se of sew a ge t r ea t m en t lea d s t o a n a ccu -m u la t ion of p h osp h or u s in t h e slu d ge. T h u s, t h e follow in g m a in com b in a t ion s a r e p ossible: 1 Liq u id , h igh ly n it r ogen ou s slu d ge, p ossibly
w it h a n in cr ea sed p h osp h or u s con t en t . 2 Dew a t er ed slu d ge, p ossibly w it h a n
in cr ea sed p h osp h or u s a n d / or lim e con t en t . 3 Dr ied slu d ge, w it h a n in cr ea sed p h
os-p h or u s con t en t .
Ta ble I sh ow s t h e aver a ge con t en t s of n u t r i-en t s in slu d ges – t h e r esea r ch w a s ca r r ied ou t b et w een 1985 a n d 1991 in N ied er sa ch sen , on e of t h e Fed er a l St a t es of Ger m a n y. Ba sed on a n aver a ge fer tilizin g va lu e of cu r r en tly r ou gh ly DM60 per ton of dr y solid m a tter a n d a n a gr i-cu ltu r a lly u tilized tota l qu a n tity of slu dge of a r ou n d 800,000 ton s of dr y solid m a tter per
a n n u m , th e a n n u a l fer tilizin g va lu e of th e slu dge su m s u p to 48 m illion DM.
N a t u r a lly, a gr icu lt u r a l u se of slu d ge is p r ofi t a ble on ly if t h er e is a n eed for t h e n u t r i-en t s w h ich a r e in clu d ed in t h e slu d ge, w h ich a r e m a in ly n it r ogen , p h osp h or u s a n d lim e. T h is n eed sh ou ld b e d et er m in ed by ca r r y in g ou t a n u t r ien t b a la n ce a n a ly sis of t h e soils on w h ich slu d ge is t o b e a p p lied , t a k in g in t o a ccou n t t h e k in d of p la n t s t o b e gr ow n on t h e soil, t h eir cr op p a t t er n , a n d t h e y ield , k in d a n d q u a n t it y of h a r vest r esid u es a n d t h e u se of ot h er fer t ilizer s.
The saving of natural resources
An – a d m it t ed ly p essim ist ic – est im a t e of glo-b a l p h osp h or ou s r esou r ces is glo-b a sed on t h e a ssu m p t ion t h a t cu r r en t r esou r ces w ill la st for a b ou t a n ot h er 500 yea r s, w it h a n a n n u a l con su m p t ion of 10 m illion t on s wor ldw id e. A t ot a l of 415,000 t on s P2O5a r e cu r r en t ly u sed in a r t ifi cia l fer t ilizer s in Ger m a n a gr icu lt u r e. Let u s fu r t h er a ssu m e t h a t t h e t ot a l a n n u a l slu d ge q u a n t it y is 3.6 m illion t on s of d r y solid m a t t er w it h a n aver a ge P2O5con t en t of 5 p er cen t , eq u a llin g 180,000 t on s of P2O5p er a n n u m . At b est , if 100 p er cen t of t h ese slu d ges w er e em p loyed in a gr icu lt u r e, t h e a m ou n t of p h osp h or u s u sed in a r t ifi cia l fer t il-izer s cou ld b e r ed u ced by m or e t h a n 40 p er cen t . T h is is sh ow n in F igu r e 1.
Table I
Co nte nt o f nutrie nts in s ludge as a pe rc e ntage o f dry, s o lid, matte r
With lime Without lime
Total N 2 .1 5 .0
NH4–N 0 .3 1 .0
P2O5 3 .1 4 .7
K2O 0 .4 0 .7
CaO 3 2 .4 5 .4
M gO 0 .8 0 .7
Po te ntial additio nal pho s phate in s ludge in c as e o f us e o f 1 0 0 % s ludge in agric ulture 1 3 2 ,0 0 0 t/ a
Ac tual Pho s phate in s ludge 1 9 9 3 / 9 4 : 4 8 ,0 0 0 t/ a Po te ntial re maining de mand in
artific ial fe rtilize r: 2 3 5 ,0 0 0 t/ a
1 9 9 3 / 9 4 : Total phosphate demand in artificial fertilizer: 4 1 5 ,0 0 0 t/ a
Figure 1
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N a t u r a lly, t h is a ssu m p t ion is a ver y id ea lized on e. St ill, it clea r ly sh ow s t h a t t h e u se of slu d ge in a gr icu lt u r e cou ld in fl u en ce t h e sav in g of n a t u r a l r esou r ces sign ifi ca n t ly.
Content of toxic substances in
sludge
Ow in g t o con t in u ou s m on it or in g of in d ir ect d isch a r ger s, t h e sew a ge t r ea t m en t a u t h or it ies h ave b een a ble t o r ed u ce t h e con t en t of n ox-iou s su b st a n ces in slu d ge sign ifi ca n t ly over t h e p a st 20 yea r s. T h e p r esen t ca d m iu m con -t en -t for exa m p le, wh ich is q u i-t e h a r m fu l -t o h u m a n s, h a s d r op p ed t o on ly 10 p er cen t of t h e con t en t 15 yea r s a go. Ta ble II illu st r a t es t h e d ecr ea se of h eav y m et a ls in slu d ge b et ween 1977 a n d 1992/ 1993.
T h e con t en t of or ga n ic t oxic su b st a n ces in slu d ge a lso r em a in s well b elow t h e lim it va lu es. An a n a ly sis of slu d ge sa m p les sh ow s, t h a t . on aver a ge, t h e a d sor b a ble or ga n ic h a lo-gen com p ou n d s (AOX) con t en t a m ou n t t o on ly 30 p er cen t , t h e p olych lor in a t ed b ip h en iles (P CB) con t en t t o 10 p er cen t a n d t h e p olych lo-r in a t ed d ioxin es/ fu lo-r a n es (P CD/ P CF ) con t en t t o 30 p er cen t of t h e r esp ect ive lim it va lu es.
It h a s to be em ph a sized th a t oth er k in ds of fer tilizer s su ch a s m in er a l fer tilizer s or a n im a l m a n u r e a lso con ta in toxic su bsta n ces. P h os-ph a te fer tilizer s, in pa r ticu la r, sh ow a n extr e-m ely h igh ca de-m iu e-m con cen tr a tion coe-m pa r ed w ith slu dge. Depen din g on th e or igin of th e ph osph or u s u sed, th e type of fer tilizer a n d th e pr ocessin g tech n ology em ployed, th e ca dm iu m con ten t ca n be a s h igh a s 300m g per k ilogr a m of P2O5. Conver sely, slu dge, h a s a n aver a ge ca dm iu m con ten t of on ly 40m g per k g P2O5. F igu r e 2 sh ow s t h e r esu lt s of som e ver y in t er -est in g r esea r ch , ca r r ied ou t over a p er iod of 30 yea r s. N o sign ifi ca n t d iffer en ce cou ld b e d et ect ed in t h e ca d m iu m con t en t of t h ose w h ea t s, w h ich a r e h igh ly sen sit ive t o h eav y m et a ls.
It goes w it h ou t say in g t h a t t h e volu m e of t oxic su b st a n ces in slu d ge m u st b e r ed u ced
even fu r t h er t o en su r e t h e lon g-t er m u se of slu d ge in a gr icu lt u r e.
Comparison of costs for sludge
disposal
T h e follow in g sp ecifi ca t ion s ca n on ly b e seen a s a r ou gh ou t lin e of t h e p r esen t sit u a t ion in Ger m a n y. A m or e exa ct cost d et er m in a t ion ca n n ot b e ca r r ied ou t w it h ou t t a k in g in t o a ccou n t t h e loca l va r ia t ion s in cost s for u se of slu d ge, or a n y ot h er d isp osa l. T h e fees for d isp osa l of slu d ge on la n d fi ll sit es, for exa m p le, a r e cu r r en t ly b et w een DM 150 a n d DM 1,500 p er t on of d r y solid m a t t er.
For t h e u se of liq u id slu d ge in a gr icu lt u r e, cost s b et w een DM 200 a n d DM 500 p er t on of d r y solid m a t t er a r e t o b e exp ect ed .
T h e t ot a l cost s for t h e u se of d ew a t er ed slu d ge in a gr icu lt u r e va r y b et w een DM 300 a n d DM 450 p er t on of d r y solid m a t t er. T h e d isp osa l of d ew a t er ed slu d ge on a la n d fi ll sit e cost s r ou gh ly DM 1,000 p er t on of d r y solid m a t t er.
T h e bu r n in g of slu d ge, in clu d in g d ew a t er -in g, d r y -in g a n d t h e d isp osa l of r esid u es on a la n d fi ll sit e cu r r en t ly cost s b et w een DM 1,000 a n d DM 2,000 p er t on of d r y solid m a t t er.
T h ese fi gu r es sh ow clea r ly t h a t t h er e is a sign ifi ca n t cost a dva n t a ge in u sin g slu d ge in a gr icu lt u r e a s op p osed t o a lt er n a t ive w ay s of d isp osa l.
Legal requirements
P r ev iou sly, a n u m er ica l list in g of t h e law s a n d r e gu la t ion s for slu d ge d isp osa l t h a t a p p lied in Ger m a n y in t h e m id d le of 1995 w a s p r esen t ed .
On 7 Oct ob er 1996, a n a d d it ion a l law, t h e “Geset z zu r För d er u n g d er Kr eisla u fw ir t sch a ft u n d zu r Sich er u n g d er u m w elt ver -t r ä glich en Besei-t igu n g von Ab fä llen ” (Law for t h e p r om ot ion of r ecyclin g a n d for sa fe-gu a r d in g a n en v ir on m en t a lly com p a t ible w a st e d isp osa l) ca m e in t o for ce. It a im s a t sav in g n a t u r a l r esou r ces a n d secu r in g en v ir on m en t a lly -com p a t ible w a st e d isp osa l.
T h is r eq u ir es avoid in g w a st e a s fa r a s p os-sible – if it ca n n ot b e avoid ed , it sh ou ld b e u sed t o p r od u ce en er gy or b e r ecycled . T h e fa ct is t h a t slu d ge ca n h a r d ly b e avoid ed , a s it a ccu m u la t es con t in u ou sly in t h e p r ocess of w a st e w a t er t r ea t m en t .
Th e “Decr ee of u se of slu dge in a gr icu ltu r e”, wh ich ca m e in to for ce in 1982, a n d w a s u pda ted in 1992, ser ves a s a ba sis for th e r ecyclin g of slu dge. Th e decr ee a im s a t pr otectin g soil, gr ou n dw a ter, pla n t, a n im a l a n d h u m a n life.
T h e d ecr ee is – in t h e fi gu r a t ive sen se – bu ilt u p on t h e follow in g seven p illa r s: Table II
Ave rage c o nte nt o f he avy me tals in s ludge , us e d in agric ulture
1977 1982 1987 1992-1993
Lead 2 9 0 1 9 0 1 5 8 c . 1 0 0
Cadmium 2 1 4 .1 3 .2 c . 2
Chromium 6 3 0 8 0 8 3 .4 c . 7 0
Copper 3 7 8 3 7 0 3 8 4 c . 2 6 0
Nickel 1 3 1 4 8 3 8 c . 3 0
M ercury 4 .8 2 .3 2 .4 c . 2
Zinc 2 ,1 4 0 1 ,4 8 0 1 ,3 0 0 c . 1 ,1 0 0
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1 It lim it s va lu es for h eav y m et a ls a n d or ga n ic t oxic com p ou n d s in slu d ge, a s sh ow n in Ta ble III.
2 Deter m in a tion of lim it va lu es w ith r ega r d to th e pr eviou s h eavy m eta l con ta m in a tion of soils; a tten tion sh ou ld be pa id to Ta ble IV. 3 N o m or e t h a n fi ve t on s of d r y solid m a t t er
m ay b e sp r ea d p er h ect a r e w it h in a p er iod of t h r ee yea r s.
4 T h e m et h od , q u a n t it y a n d t im e for t h e la n d a p p lica t ion of slu d ge sh ou ld b e a d ju st ed t o t h e n u t r ien t n eed of t h e p la n t s con cer n ed , a lso t h e n a t u r a l n u t r ien t r esou r ces a n d or ga n ic su b st a n ces w it h in t h e soil a s well a s t h e sp ecifi c con d it ion s for cu lt iva t ion . 5 T h e lim it va lu es sp ecifi ed in t h e d ecr ee
r e p r esen t t h e m a xim u m a llow a ble; a ct u a l va lu es sh ou ld b e a s low a s p ossible. 6 Th e la n d a pplica tion of slu dge is on ly le ga l
on a ppr opr ia te a r a ble la n d. It is n ot a llowed for in sta n ce w ith in n a tu r a l r eser ves, n a tion a l pa r k s, w a ter pr otection zon es, etc. 7 T h e la n d a p p lica t ion of slu d ge n eed s t o b e
a u t h or ised a n d con t r olled by t h e r eleva n t a u t h or it ies.
The use of sludge in agriculture
requires team-work
T h e m a in in st it u t ion s con cer n ed a r e list ed a s follow s:
• t h e sew a ge t r ea t m en t a u t h or it y, w h ich is r esp on sible for t h e q u a lit y of t h e slu d ge; • t h e fa r m er, w h o sp r ea d s t h e slu d ge on
a p p r op r ia t e a r ea s;
• t h e con su lt a n t , w h o in fl u en ces t h e p r ocess t ech n ology of t h e t r ea t m en t p la n t a n d d esign s t h e la n d a p p lica t ion sch em e;
• t h e la b or a t or y, w h er e t h e slu d ge a n d t h e soil sa m p les of t h e a r ea s con cer n ed a r e a n a ly sed ;
• t h e b iosolid s la n d a p p lica t ion en t er p r ise, w h ich or ga n izes t h e t r a n sp or t a n d d ist r ibu -t ion of slu d ge;
• t h e su p er v isor y a u t h or it y, wh ich ch eck s com p lia n ce w it h t h e d ecr ee of u sin g slu d ge in a ccor d a n ce w it h t h e a u t h or it ies involved .
Steps towards the realization of
using sludge in agriculture
T h e follow in g list s in d ica t e t h e st e p s t h a t sh ou ld b e t a k en t o en su r e t h e u se of slu d ge in a gr icu lt u r e.
1 P r e p a r a t or y Mea su r es
• In for m a t ion ca m p a ign / aw a r en ess r a isin g in a gr icu lt u r e.
• P r eselect ion of a r ea s, exclu sion of p r oh ib it ed a r ea s.
• Sa m p lin g a n d a n a ly sin g of soil. • In t er p r et a t ion of soil a n a ly sis (h eav y
m et a ls!)
• E la b or a t ion of m a n u r in g p la n . • Select ion of slu d ge, d et er m in a t ion of
q u a n t it ies, fi xa t ion of d a t e. • Risk m a n a gem en t , lia b ilit y. • P r e p a r a t ion of d eliver y n ot es. • An n ou n cem en t t o su p er v isin g
a u t h or it y
• Det er m in a t ion of t r a n sp or t , in t er m ed ia t e st or a ge, d ist r ibu t ion m od a lit ies.
• Ba la n cin g ou t of slu d ge d em a n d / slu d ge d isp osa l.
2 Im p lem en t a t ion
• T r a n sp or t a t ion / in t er m ed ia t e st or a ge. • Dist r ibu t ion .
• Mon it or in g. Figure 2
Co nte nt o f c admium in grain o f whe at/ s traw o f whe at
0 ,3 5 0 3 0 ,2 5 0 2 0 ,1 5 0 1 0 ,0 5 0
Grain of wheat
Straw
MF D SQ1 SQ2 SQ3 MF D SQ1 SQ2 SQ3
Key
MF ... mine ral fe rtilize r D ... dung
SQ1 ... s ludge quantity ac c . to “ De c re e o f us e o f s ludge in agric ulture ” SQ2 ... s ludge quantity duplic ate d
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3 Docu m en t a t ion
• F ill u p d eliver y n ot es.
• Dist r ibu t e d eliver y n ot es t o a ll p a r t ies in volved
• Ca r t ogr a p h ica l m on it or in g of a r ea s. • E st a blish ca r d -in d ex, in for m
su p er v isin g a u t h or it y
Liability
In sp it e of t h e gr ea t est p ossible ca r e a n d com -p lia n ce w it h t h e Decr ee of Use of Slu d ge, t h er e a r e a few h id d en r isk s in volved wh ich ca n n ot a lw ay s b e elim in a t ed a s ea r ly a s a t t h e st a ge of la n d a p p lica t ion . Th u s, t h e so-ca lled “Wa st e w a t er slu d ge fu n d ” w a s fou n d ed in 1990 on a volu n t a r y b a sis. T h e fu n d is fi n a n ced by con t r ibu t ion s fr om t h e sew a ge t r ea t m en t a u t h or it ies wh ich a m ou n t t o DM 62 m illion (in 1995). Th e fu n d cu r r en t ly h a s a r ou n d 800 m em b er s, a ll of t h em sew a ge t r ea t m en t a u t h or it ies. T h e m a xim u m p ossible cla im for a sin gle d a m a ge is DM 2 m illion .
Ap a r t fr om a few goodw ill ca ses (w it h a t ot a l com p en sa t ion of DM 26,000 in 1995), n o cla im s for d a m a ges h ave b een m a d e so fa r.
Conclusion
Ow in g t o a n in cr ea sin g t igh t en in g-u p of law s a n d r e gu la t ion s, a p r ogr essive im p r ovem en t a n d r efi n em en t of a dva n ced w a st ew a t er t r ea t -m en t t ech n ologies a n d – la st bu t n ot lea st – a con t in u ou s m on it or in g of in d ir ect d isch a r g-er s, t h e sew a ge t r ea t m en t a u t h or it ies h ave b een a ble t o r ed u ce t h e con t en t of n oxiou s or ga n ic a n d in or ga n ic su b st a n ces in slu d ge sign ifi ca n t ly over t h e p a st 20 yea r s. Con seq u en t ia lly, slu d ge fr om w a st ew a t er t r ea t m en t p la n t s ca n b e a p p lied in a gr icu l-t u r e su ccessfu lly a n d w il-t h ou l-t a n y r isk if cer t a in st a n d a r d s – e.g. h ygien e, p r ev iou s a n d p er m issible a d d it ion a l soil loa d levels – a r e t o b e m et , if su it a ble cr op s a n d b a la n ced cr op p a t t er n s a r e ch osen a n d if a n op er a t ion -a lly efficien t logist ics, -a d m in ist r -a t ion -a n d cost m a n a gem en t sy st em is ava ila ble.
T h u s, t h e u se of slu d ge in a gr icu lt u r e con -t r ib u -t es q u i-t e con sid er a b ly -t o -t h e id ea of r ecir cu la t in g of va lu a ble su b st a n ces in t o n a t u r e a n d h elp s sav in g of n a t u r a l r esou r ces.
Further reading
E sch , B., Kr ü ger, G. a n d La n gen oh l, T. (1996), L a n d w ir tsch a ftlich e K lä rsch la m m v er w er tu n g (u se of slu d ge in a g r icu ltu re), ATVIn for m a -t ion , Abw a sser -t ech n isch e Ver ein igu n g, H en n ef, Ger m a n y.
Gesetz z u r Förd eru n g d er K reisla u fw ir tsch a ft u n d S ich eru n g d er u m w eltv er trä glich en B eseiti-gu n g v on A b fä llen (la w for th e p r om otion of recyclin g a n d for sa fegu a rd in g a n en v ir on m en -ta lly com p a tible w a ste d isp osa l)(1994), Bu n d esgeset zbla t t Teil I, Bon n , p p. 2705-728 (com in g in t o for ce on 7 Oct ob er 1996). K lä rsch la m m v er ord n u n g (d ecree of u se of slu d ge in
a g r icu ltu re) (1992), Bu n d esgeset zbla t t Teil I, Bon n , p p. 912-34 (com in g in t o for ce on 1 J u ly 1992).
Lesch b er, R., Ba n n ick , C.G. (1995), S ch la m m ch a ra k ter isieru n g, b eh a n d lu n g u n d v er w en -d u n g (slu -d ge ch a ra ter iz a tion , trea tm en t a n -d u se), Kor r esp on d en z Abw a sser, H eft 11, Abw a sser t ech n isch e Ver ein igu n g, H en n ef, Ger m a n y.
Ma tth ew s, P. (1996),A Glob a l A tla s of Wa stew a ter S lu d ge a n d B iosolid s Use a n d Disposa l, Scien -tific a n d Tech n ica l Repor t No. 4, In ter n a tion a l Associa tion on Wa ter Qu a lity, Lon don .
Table III
Limit value s in s ludge
Limit values in sludge (mg/ kg dry solid matter)
Parameter Light soils Normal soils
Lead 9 0 0 9 0 0
Cadmium) 5 1 0
Chromium 9 0 0 9 0 0
Copper 8 0 0 8 0 0
Nickel 2 0 0 2 0 0
M ercury 8 8
Zinc 2 ,0 0 0 2 ,0 0 0
PCP 0 .2 mg/ kg dry solid matter
Dioxines/ furanes 1 0 0 ng TE/ kg dry solid matter
AOX 5 0 0 mg/ kg dry solid matter
Table IV
Limit value s in s o il (mg/ kg dry s o lid matte r)
Lead 1 0 0
Cadmium 1 .5 (*1 )
Chromium 1 0 0
Copper 6 0
Nickel 5 0
M ercury 1
