Hydrocarbon Engineering - July 2020

47 / 76

remaining free water and significant amounts of interstitial
water to be pressed out of the solid cake.
Significant efforts are paid to the high pressure section
when troubleshooting and optimising BFPs; however, the GDS
removes the majority of the water in terms of lb and can have
the largest impact in terms of performance of the other two
sections downstream of it. As seen in Figure 1, an increase in
sludge concentration from 1% to 2% results in 50.5% reduction
of water remaining in the sludge. Properly operating and
optimised GDSs can double, triple, or even more so increase
the solids concentration in the sludge prior to the wedge
compression section. It is common to see solids percentages
ranging from 5 – 7% after effective treatment in the
GDS. If the incoming sludge contained 2% solids,
achieving a 5% solids concentration entering into the
wedge section means a 61% reduction in lb and
volume seen by the wedge compression and high
pressure sections. A 7% solids concentration means a
73% reduction. These significant reductions in free
water entering the wedge compression and high
pressure sections means that the solids are less likely
to press out the side of the belt and that the press
utilises more of the energy imparted on the sludge
to press out interstitial water, as the free water is not
present.
Optimising the GDS
Optimising the GDS generally consists of two
modifications:
1. Altering sludge distribution to ensure even
placement of sludge onto the GDS.
2. Altering the plows to maximise belt available for
free water drainage.
By ensuring sludge distribution onto the gravity
section occurs in an even, consistent manner, solids
build-up will be equal across the entirety of the belt.
This even distribution will result in almost uniform
thickness of solids across the whole belt and help
utilise the entire surface area available for water to
drain off the solids. When solids build up in thicker
layers, typically occurring in the centre of the belt,
the edges of the feed will dewater effectively, but
the increased depth of solids that free water must
drain through prevent optimised gravity drainage of
water for significant portions of the sludge. The
increased carryover of free water results in increased
mass of water requiring removal in the wedge and
high pressure sections, and will ultimately result in
uneven pressure applied across the pressure sections.
Modifications to the distribution system often do
not require major expenditures or additions to BFPs.
One site utilised a slotted PVC pipe to distribute the sludge to
the GDS. The pipe was only half the width of the belt, meaning
about a quarter of the belts width on each side of the
distribution pipe saw little to no solids under normal feed rates.
The optimisation of this distribution system consisted of
replacing the pipe with one that extended to 2 in. away from
the edges of the GDS, resulting in even distribution of sludge
feed across the entirety of the belt.
Even with proper and uniform distribution of treated sludge
across the GDS of a BFP, water can pool on top of the sludge
and not properly drain through the solids now laying on the
belt. In order to combat this, many BFPs utilise plows to shift the
Table 2. % improvement realised by installing plows into the gravity drainage section of the belt filter
press
Cake
solids
lb solids/t
cake
lb H2O/t
cake
t cake required
to remove 1 t
solids
% reduction in t
disposed of with 1.8%
cake increase
lb H2O shipped
to remove 1 t
solids
% reduction in lb H2O
removed with 1.8% cake
increase
12.68% 253.6 1746.4 7.89 13 773
14.49% 289.8 1710.2 6.90 12.49% 11 803 14.31%
20 Years and Counting...
the Best Protection
for People and Equipment
Blast Resistant
Modular
Buildings
Perimeter
Security
Products
Protective
Doors
& Accessories
Site
Services