Environmental Engineering Reference
In-Depth Information
and some organic pollutants may restrict the use of biosolids for fertilizer (Aparicio et al.). If the
pollutants are below the limits, biosolids can be beneficially used for fertilizers and soil enhancers.
Biosolids can be used directly (some unpleasant odor problems may arise) or additionally composted
to produce odorless soil like material. Anaerobically digested sludge still has enough organic
material present to be composted (autothermally at 60°C) (Cukjati 2008). Subsequent sections
present detailed information about composting.
Polluted biosolids are mostly incinerated or co-incinerated with other fuels. It is possible to
recover some energy from the sludge incineration process. The main problem with incineration
is low combustion value of dewatered wet sludge (Figure 32.13). Although dry sludge has the
combustion value (CV) of 15-22 MJ/kg (fresh) or 8-15 MJ/kg (digested), it contains more than 50%
water, which reduces the combustion value significantly. Because of sludge composition (cellular
material) it is extremely hard and energy demanding to dewater sludge mechanically to values more
than 40%. Therefore, sludge must be thermally dried, and this significantly reduces the efficiency
of sludge incineration process. To improve the incineration process, some of the heat necessary for
sludge drying may be regenerated within the incineration process.
Better efficiency of sludge treatment can also be achieved with modern thermal processes, either
low temperature (Vieira et al. 2009) or high temperature method, like pyrolysis (Kim and Parker
2008; Hossain et al. 2009), producing many useful fuels.
The liquid fraction of sludge dewatering (sludge supernatant) must also be treated. It is rich with
nutrients (total nitrogen over 1500 mg/L) and poor in COD (200-300 mg/L), and therefore a direct
use is not advisable. Sludge supernatant is usually returned to the WWTP inflow and mixed with
raw wastewater. Sludge supernatant does not present a significant portion in the volume flow or the
COD/BOD load of the raw wastewater. However, it does present approximately. 50% of the WWTP
ammonium load (30-40% total nitrogen load) (Table 32.20). The consequences of this fact are
16
0.3
CV 15 MJkg
-1
14
0.25
12
0.2
10
0.15
8
6
0.1
4
0.05
2
0
0
5
10
15
20
25
00.1
0.2
0.3
0.4
0.5
0.60.7
0.80.9
1
CV [MJkg
-1
]
% of water
FIGure 32.13
Combustion value (CV) and limit of combustibility of dewatered municipal sludge.
taBle 32.20
ammonium in the WWtP (case of 200,000 Pe WWtP)
average
ammonium
concentration
(mg/l)
Percent of
ammonium
load
Flow
(m
3
/year)
ammonium
(g)
Percent
of Flow
Raw wastewater influent
6,744,312
10.6
71,489,707
99.3
49.9
Sludge supernatant
48,112
1487
71,542,544
0.7
50.1
Source:
Zupancic, G.D. and Ros, M., Ammonia removal in sludge digestion utilizing nitrificationwith pure oxygen
aeration.
Nutrient Management in Wastewater Treatment Processes and Recycle Streams
: IWA specialized
conference, Krakow, Poland, IWA Publishing, London, 2005.
